refactor: localesをworkspace管理下のパッケージに (#16895)

* refactor: localesをworkspace管理下のパッケージに

* fix copilot review

* move

* move

* rename

* fix ci

* revert unwanted indent changes

* fix

* fix

* fix

* fix

* 間違えてコミットしていたのを戻す

* 不要

* 追加漏れ

* ymlの場所だけ戻す

* localesの位置を戻したのでこの差分は不要

* 内容的にlocalesにある方が正しい

* i18nパッケージ用のREADME.mdを用意

* fix locale.yml

* fix locale.yml

---------

Co-authored-by: kakkokari-gtyih <67428053+kakkokari-gtyih@users.noreply.github.com>

51 files changed, 860 insertions, 519 deletions

diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index d80c56f032..33a1ccbc76 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -111,10 +111,5 @@ jobs:
         node-version-file: '.node-version'
         cache: 'pnpm'
     - run: pnpm i --frozen-lockfile
-    - run: pnpm --filter misskey-js run build
-      if: ${{ matrix.workspace == 'backend' || matrix.workspace == 'frontend' || matrix.workspace == 'sw' }}
-    - run: pnpm --filter misskey-reversi run build
-      if: ${{ matrix.workspace == 'backend' || matrix.workspace == 'frontend' }}
-    - run: pnpm --filter misskey-bubble-game run build
-      if: ${{ matrix.workspace == 'frontend' }}
+    - run: pnpm --filter "${{ matrix.workspace }}^..." run build
     - run: pnpm --filter ${{ matrix.workspace }} run typecheck
diff --git a/.github/workflows/locale.yml b/.github/workflows/locale.yml
index e63d83997b..d75335f38f 100644
--- a/.github/workflows/locale.yml
+++ b/.github/workflows/locale.yml
@@ -3,10 +3,12 @@ name: Lint
 on:
   push:
     paths:
+      - packages/i18n/**
       - locales/**
       - .github/workflows/locale.yml
   pull_request:
     paths:
+      - packages/i18n/**
       - locales/**
       - .github/workflows/locale.yml
 jobs:
@@ -14,15 +16,18 @@ jobs:
     runs-on: ubuntu-latest
     continue-on-error: true
     steps:
-    - uses: actions/checkout@v4.3.0
-      with:
-        fetch-depth: 0
-        submodules: true
-    - name: Setup pnpm
-      uses: pnpm/action-setup@v4.2.0
-    - uses: actions/setup-node@v4.4.0
-      with:
-        node-version-file: '.node-version'
-        cache: 'pnpm'
-    - run: pnpm i --frozen-lockfile
-    - run: cd locales && node verify.js
+      - uses: actions/checkout@v4.3.0
+        with:
+          fetch-depth: 0
+          submodules: true
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4.2.0
+      - uses: actions/setup-node@v4.4.0
+        with:
+          node-version-file: ".node-version"
+          cache: "pnpm"
+      - run: pnpm i --frozen-lockfile
+      - run: pnpm --filter i18n build
+      - name: Verify Locales
+        working-directory: ./packages/i18n
+        run: pnpm run verify
diff --git a/Dockerfile b/Dockerfile
index 20e24d1dc2..a071970927 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -24,6 +24,7 @@ COPY --link ["packages/frontend-shared/package.json", "./packages/frontend-share
 COPY --link ["packages/frontend/package.json", "./packages/frontend/"]
 COPY --link ["packages/frontend-embed/package.json", "./packages/frontend-embed/"]
 COPY --link ["packages/frontend-builder/package.json", "./packages/frontend-builder/"]
+COPY --link ["packages/i18n/package.json", "./packages/i18n/"]
 COPY --link ["packages/icons-subsetter/package.json", "./packages/icons-subsetter/"]
 COPY --link ["packages/sw/package.json", "./packages/sw/"]
 COPY --link ["packages/misskey-js/package.json", "./packages/misskey-js/"]
@@ -101,6 +102,7 @@ COPY --chown=misskey:misskey --from=native-builder /misskey/packages/misskey-js/
 COPY --chown=misskey:misskey --from=native-builder /misskey/packages/misskey-reversi/built ./packages/misskey-reversi/built
 COPY --chown=misskey:misskey --from=native-builder /misskey/packages/misskey-bubble-game/built ./packages/misskey-bubble-game/built
 COPY --chown=misskey:misskey --from=native-builder /misskey/packages/backend/built ./packages/backend/built
+COPY --chown=misskey:misskey --from=native-builder /misskey/packages/i18n/built ./packages/i18n/built
 COPY --chown=misskey:misskey --from=native-builder /misskey/fluent-emojis /misskey/fluent-emojis
 COPY --chown=misskey:misskey . ./
 
diff --git a/locales/generateDTS.js b/locales/generateDTS.js
deleted file mode 100644
index ab0613cc82..0000000000
--- a/locales/generateDTS.js
+++ /dev/null
@@ -1,232 +0,0 @@
-import * as fs from 'node:fs';
-import { fileURLToPath } from 'node:url';
-import { dirname } from 'node:path';
-import * as yaml from 'js-yaml';
-import ts from 'typescript';
-
-const __filename = fileURLToPath(import.meta.url);
-const __dirname = dirname(__filename);
-const parameterRegExp = /\{(\w+)\}/g;
-
-function createMemberType(item) {
-	if (typeof item !== 'string') {
-		return ts.factory.createTypeLiteralNode(createMembers(item));
-	}
-	const parameters = Array.from(
-		item.matchAll(parameterRegExp),
-		([, parameter]) => parameter,
-	);
-	return parameters.length
-		? ts.factory.createTypeReferenceNode(
-				ts.factory.createIdentifier('ParameterizedString'),
-				[
-					ts.factory.createUnionTypeNode(
-						parameters.map((parameter) =>
-							ts.factory.createStringLiteral(parameter),
-						),
-					),
-				],
-			)
-		: ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword);
-}
-
-function createMembers(record) {
-	return Object.entries(record).map(([k, v]) => {
-		const node = ts.factory.createPropertySignature(
-			undefined,
-			ts.factory.createStringLiteral(k),
-			undefined,
-			createMemberType(v),
-		);
-		if (typeof v === 'string') {
-			ts.addSyntheticLeadingComment(
-				node,
-				ts.SyntaxKind.MultiLineCommentTrivia,
-				`*
- * ${v.replace(/\n/g, '\n * ')}
- `,
-				true,
-			);
-		}
-		return node;
-	});
-}
-
-export default function generateDTS() {
-	const locale = yaml.load(fs.readFileSync(`${__dirname}/ja-JP.yml`, 'utf-8'));
-	const members = createMembers(locale);
-	const elements = [
-		ts.factory.createVariableStatement(
-			[ts.factory.createToken(ts.SyntaxKind.DeclareKeyword)],
-			ts.factory.createVariableDeclarationList(
-				[
-					ts.factory.createVariableDeclaration(
-						ts.factory.createIdentifier('kParameters'),
-						undefined,
-						ts.factory.createTypeOperatorNode(
-							ts.SyntaxKind.UniqueKeyword,
-							ts.factory.createKeywordTypeNode(ts.SyntaxKind.SymbolKeyword),
-						),
-						undefined,
-					),
-				],
-				ts.NodeFlags.Const,
-			),
-		),
-		ts.factory.createTypeAliasDeclaration(
-			[ts.factory.createToken(ts.SyntaxKind.ExportKeyword)],
-			ts.factory.createIdentifier('ParameterizedString'),
-			[
-				ts.factory.createTypeParameterDeclaration(
-					undefined,
-					ts.factory.createIdentifier('T'),
-					ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword),
-					ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword),
-				),
-			],
-			ts.factory.createIntersectionTypeNode([
-				ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword),
-				ts.factory.createTypeLiteralNode([
-					ts.factory.createPropertySignature(
-						undefined,
-						ts.factory.createComputedPropertyName(
-							ts.factory.createIdentifier('kParameters'),
-						),
-						undefined,
-						ts.factory.createTypeReferenceNode(
-							ts.factory.createIdentifier('T'),
-							undefined,
-						),
-					),
-				])
-			]),
-		),
-		ts.factory.createInterfaceDeclaration(
-			[ts.factory.createToken(ts.SyntaxKind.ExportKeyword)],
-			ts.factory.createIdentifier('ILocale'),
-			undefined,
-			undefined,
-			[
-				ts.factory.createIndexSignature(
-					undefined,
-					[
-						ts.factory.createParameterDeclaration(
-							undefined,
-							undefined,
-							ts.factory.createIdentifier('_'),
-							undefined,
-							ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword),
-							undefined,
-						),
-					],
-					ts.factory.createUnionTypeNode([
-						ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword),
-						ts.factory.createTypeReferenceNode(
-							ts.factory.createIdentifier('ParameterizedString'),
-						),
-						ts.factory.createTypeReferenceNode(
-							ts.factory.createIdentifier('ILocale'),
-							undefined,
-						),
-					]),
-				),
-			],
-		),
-		ts.factory.createInterfaceDeclaration(
-			[ts.factory.createToken(ts.SyntaxKind.ExportKeyword)],
-			ts.factory.createIdentifier('Locale'),
-			undefined,
-			[
-				ts.factory.createHeritageClause(ts.SyntaxKind.ExtendsKeyword, [
-					ts.factory.createExpressionWithTypeArguments(
-						ts.factory.createIdentifier('ILocale'),
-						undefined,
-					),
-				]),
-			],
-			members,
-		),
-		ts.factory.createVariableStatement(
-			[ts.factory.createToken(ts.SyntaxKind.DeclareKeyword)],
-			ts.factory.createVariableDeclarationList(
-				[
-					ts.factory.createVariableDeclaration(
-						ts.factory.createIdentifier('locales'),
-						undefined,
-						ts.factory.createTypeLiteralNode([
-							ts.factory.createIndexSignature(
-								undefined,
-								[
-									ts.factory.createParameterDeclaration(
-										undefined,
-										undefined,
-										ts.factory.createIdentifier('lang'),
-										undefined,
-										ts.factory.createKeywordTypeNode(
-											ts.SyntaxKind.StringKeyword,
-										),
-										undefined,
-									),
-								],
-								ts.factory.createTypeReferenceNode(
-									ts.factory.createIdentifier('Locale'),
-									undefined,
-								),
-							),
-						]),
-						undefined,
-					),
-				],
-				ts.NodeFlags.Const,
-			),
-		),
-		ts.factory.createFunctionDeclaration(
-			[ts.factory.createModifier(ts.SyntaxKind.ExportKeyword)],
-			undefined,
-			ts.factory.createIdentifier('build'),
-			undefined,
-			[],
-			ts.factory.createTypeReferenceNode(
-				ts.factory.createIdentifier('Locale'),
-				undefined,
-			),
-			undefined,
-		),
-		ts.factory.createExportDefault(ts.factory.createIdentifier('locales')),
-	];
-	ts.addSyntheticLeadingComment(
-		elements[0],
-		ts.SyntaxKind.MultiLineCommentTrivia,
-		' eslint-disable ',
-		true,
-	);
-	ts.addSyntheticLeadingComment(
-		elements[0],
-		ts.SyntaxKind.SingleLineCommentTrivia,
-		' This file is generated by locales/generateDTS.js',
-		true,
-	);
-	ts.addSyntheticLeadingComment(
-		elements[0],
-		ts.SyntaxKind.SingleLineCommentTrivia,
-		' Do not edit this file directly.',
-		true,
-	);
-	const printed = ts
-		.createPrinter({
-			newLine: ts.NewLineKind.LineFeed,
-		})
-		.printList(
-			ts.ListFormat.MultiLine,
-			ts.factory.createNodeArray(elements),
-			ts.createSourceFile(
-				'index.d.ts',
-				'',
-				ts.ScriptTarget.ESNext,
-				true,
-				ts.ScriptKind.TS,
-			),
-		);
-
-	fs.writeFileSync(`${__dirname}/index.d.ts`, printed, 'utf-8');
-}
diff --git a/locales/index.js b/locales/index.js
deleted file mode 100644
index 6d9cf4796b..0000000000
--- a/locales/index.js
+++ /dev/null
@@ -1,93 +0,0 @@
-/**
- * Languages Loader
- */
-
-import * as fs from 'node:fs';
-import * as yaml from 'js-yaml';
-
-const merge = (...args) => args.reduce((a, c) => ({
-	...a,
-	...c,
-	...Object.entries(a)
-		.filter(([k]) => c && typeof c[k] === 'object')
-		.reduce((a, [k, v]) => (a[k] = merge(v, c[k]), a), {})
-}), {});
-
-const languages = [
-	'ar-SA',
-	'ca-ES',
-	'cs-CZ',
-	'da-DK',
-	'de-DE',
-	'en-US',
-	'es-ES',
-	'fr-FR',
-	'id-ID',
-	'it-IT',
-	'ja-JP',
-	'ja-KS',
-	'kab-KAB',
-	'kn-IN',
-	'ko-KR',
-	'nl-NL',
-	'no-NO',
-	'pl-PL',
-	'pt-PT',
-	'ru-RU',
-	'sk-SK',
-	'th-TH',
-	'tr-TR',
-	'ug-CN',
-	'uk-UA',
-	'vi-VN',
-	'zh-CN',
-	'zh-TW',
-];
-
-const primaries = {
-	'en': 'US',
-	'ja': 'JP',
-	'zh': 'CN',
-};
-
-// 何故か文字列にバックスペース文字が混入することがあり、YAMLが壊れるので取り除く
-const clean = (text) => text.replace(new RegExp(String.fromCodePoint(0x08), 'g'), '');
-
-export function build() {
-	// vitestの挙動を調整するため、一度ローカル変数化する必要がある
-	// https://github.com/vitest-dev/vitest/issues/3988#issuecomment-1686599577
-	// https://github.com/misskey-dev/misskey/pull/14057#issuecomment-2192833785
-	const metaUrl = import.meta.url;
-	const locales = languages.reduce((a, c) => (a[c] = yaml.load(clean(fs.readFileSync(new URL(`${c}.yml`, metaUrl), 'utf-8'))) || {}, a), {});
-
-	// 空文字列が入ることがあり、フォールバックが動作しなくなるのでプロパティごと消す
-	const removeEmpty = (obj) => {
-		for (const [k, v] of Object.entries(obj)) {
-			if (v === '') {
-				delete obj[k];
-			} else if (typeof v === 'object') {
-				removeEmpty(v);
-			}
-		}
-		return obj;
-	};
-	removeEmpty(locales);
-
-	return Object.entries(locales)
-		.reduce((a, [k, v]) => (a[k] = (() => {
-			const [lang] = k.split('-');
-			switch (k) {
-				case 'ja-JP': return v;
-				case 'ja-KS':
-				case 'en-US': return merge(locales['ja-JP'], v);
-				default: return merge(
-					locales['ja-JP'],
-					locales['en-US'],
-					locales[`${lang}-${primaries[lang]}`] ?? {},
-					v
-				);
-			}
-		})(), a), {});
-}
-
-export default build();
diff --git a/locales/package.json b/locales/package.json
deleted file mode 100644
index bedb411a91..0000000000
--- a/locales/package.json
+++ /dev/null
@@ -1,3 +0,0 @@
-{
-	"type": "module"
-}
diff --git a/locales/verify.js b/locales/verify.js
deleted file mode 100644
index a8e9875d6e..0000000000
--- a/locales/verify.js
+++ /dev/null
@@ -1,53 +0,0 @@
-import locales from './index.js';
-
-let valid = true;
-
-function writeError(type, lang, tree, data) {
-	process.stderr.write(JSON.stringify({ type, lang, tree, data }));
-	process.stderr.write('\n');
-	valid = false;
-}
-
-function verify(expected, actual, lang, trace) {
-	for (let key in expected) {
-		if (!Object.prototype.hasOwnProperty.call(actual, key)) {
-			continue;
-		}
-		if (typeof expected[key] === 'object') {
-			if (typeof actual[key] !== 'object') {
-				writeError('mismatched_type', lang, trace ? `${trace}.${key}` : key, { expected: 'object', actual: typeof actual[key] });
-				continue;
-			}
-			verify(expected[key], actual[key], lang, trace ? `${trace}.${key}` : key);
-		} else if (typeof expected[key] === 'string') {
-			switch (typeof actual[key]) {
-				case 'object':
-					writeError('mismatched_type', lang, trace ? `${trace}.${key}` : key, { expected: 'string', actual: 'object' });
-					break;
-				case 'undefined':
-					continue;
-				case 'string':
-					const expectedParameters = new Set(expected[key].match(/\{[^}]+\}/g)?.map((s) => s.slice(1, -1)));
-					const actualParameters = new Set(actual[key].match(/\{[^}]+\}/g)?.map((s) => s.slice(1, -1)));
-					for (let parameter of expectedParameters) {
-						if (!actualParameters.has(parameter)) {
-							writeError('missing_parameter', lang, trace ? `${trace}.${key}` : key, { parameter });
-						}
-					}
-			}
-		}
-	}
-}
-
-const { ['ja-JP']: original, ...verifiees } = locales;
-
-for (let lang in verifiees) {
-	if (!Object.prototype.hasOwnProperty.call(locales, lang)) {
-		continue;
-	}
-	verify(original, verifiees[lang], lang);
-}
-
-if (!valid) {
-	process.exit(1);
-}
diff --git a/package.json b/package.json
index 664c5e9e71..f11b24d0ad 100644
--- a/package.json
+++ b/package.json
@@ -8,15 +8,17 @@
 	},
 	"packageManager": "pnpm@10.22.0",
 	"workspaces": [
-		"packages/frontend-shared",
-		"packages/frontend",
-		"packages/frontend-embed",
-		"packages/icons-subsetter",
-		"packages/backend",
-		"packages/sw",
 		"packages/misskey-js",
+		"packages/i18n",
 		"packages/misskey-reversi",
-		"packages/misskey-bubble-game"
+		"packages/misskey-bubble-game",
+		"packages/icons-subsetter",
+		"packages/frontend-shared",
+		"packages/frontend-builder",
+		"packages/sw",
+		"packages/backend",
+		"packages/frontend",
+		"packages/frontend-embed"
 	],
 	"private": true,
 	"scripts": {
@@ -68,6 +70,7 @@
 	},
 	"devDependencies": {
 		"@eslint/js": "9.39.1",
+		"i18n": "workspace:*",
 		"@misskey-dev/eslint-plugin": "2.2.0",
 		"@types/js-yaml": "4.0.9",
 		"@types/node": "24.10.1",
diff --git a/packages/backend/src/core/NotificationService.ts b/packages/backend/src/core/NotificationService.ts
index eeade4569b..310ffec7ce 100644
--- a/packages/backend/src/core/NotificationService.ts
+++ b/packages/backend/src/core/NotificationService.ts
@@ -202,7 +202,7 @@ export class NotificationService implements OnApplicationShutdown {
 	}
 
 	// TODO
-	//const locales = await import('../../../../locales/index.js');
+	//const locales = await import('i18n');
 
 	// TODO: locale ファイルをクライアント用とサーバー用で分けたい
 
@@ -271,7 +271,7 @@ export class NotificationService implements OnApplicationShutdown {
 		let untilTime = untilId ? this.toXListId(untilId) : null;
 
 		let notifications: MiNotification[];
-		for (;;) {
+		for (; ;) {
 			let notificationsRes: [id: string, fields: string[]][];
 
 			// sinceidのみの場合は古い順、そうでない場合は新しい順。 QueryService.makePaginationQueryも参照
diff --git a/packages/frontend-builder/locale-inliner.ts b/packages/frontend-builder/locale-inliner.ts
index 9bef465eeb..191d7250a6 100644
--- a/packages/frontend-builder/locale-inliner.ts
+++ b/packages/frontend-builder/locale-inliner.ts
@@ -10,7 +10,7 @@ import { collectModifications } from './locale-inliner/collect-modifications.js'
 import { applyWithLocale } from './locale-inliner/apply-with-locale.js';
 import { blankLogger } from './logger.js';
 import type { Logger } from './logger.js';
-import type { Locale } from '../../locales/index.js';
+import type { Locale } from 'i18n';
 import type { Manifest as ViteManifest } from 'vite';
 
 export class LocaleInliner {
diff --git a/packages/frontend-builder/locale-inliner/apply-with-locale.ts b/packages/frontend-builder/locale-inliner/apply-with-locale.ts
index 5e601cdf12..78851d3029 100644
--- a/packages/frontend-builder/locale-inliner/apply-with-locale.ts
+++ b/packages/frontend-builder/locale-inliner/apply-with-locale.ts
@@ -5,7 +5,7 @@
 
 import MagicString from 'magic-string';
 import { assertNever } from '../utils.js';
-import type { Locale, ILocale } from '../../../locales/index.js';
+import type { ILocale, Locale } from 'i18n';
 import type { TextModification } from '../locale-inliner.js';
 import type { Logger } from '../logger.js';
 
diff --git a/packages/frontend-builder/package.json b/packages/frontend-builder/package.json
index d01e4c86ed..37dd133fe6 100644
--- a/packages/frontend-builder/package.json
+++ b/packages/frontend-builder/package.json
@@ -18,6 +18,7 @@
 		"typescript": "5.9.3"
 	},
 	"dependencies": {
+		"i18n": "workspace:*",
 		"estree-walker": "3.0.3",
 		"magic-string": "0.30.21",
 		"vite": "7.2.4"
diff --git a/packages/frontend-embed/build.ts b/packages/frontend-embed/build.ts
index 737233a4d0..4e1f588802 100644
--- a/packages/frontend-embed/build.ts
+++ b/packages/frontend-embed/build.ts
@@ -2,7 +2,7 @@ import * as fs from 'fs/promises';
 import url from 'node:url';
 import path from 'node:path';
 import { execa } from 'execa';
-import locales from '../../locales/index.js';
+import locales from 'i18n';
 import { LocaleInliner } from '../frontend-builder/locale-inliner.js'
 import { createLogger } from '../frontend-builder/logger';
 
diff --git a/packages/frontend-embed/package.json b/packages/frontend-embed/package.json
index 85e25c8faa..c27583cf86 100644
--- a/packages/frontend-embed/package.json
+++ b/packages/frontend-embed/package.json
@@ -11,6 +11,7 @@
 	},
 	"dependencies": {
 		"@discordapp/twemoji": "16.0.1",
+		"i18n": "workspace:*",
 		"@rollup/plugin-json": "6.1.0",
 		"@rollup/plugin-replace": "6.0.3",
 		"@rollup/pluginutils": "5.3.0",
diff --git a/packages/frontend-embed/src/components/I18n.vue b/packages/frontend-embed/src/components/I18n.vue
index b621110ec9..9866e50958 100644
--- a/packages/frontend-embed/src/components/I18n.vue
+++ b/packages/frontend-embed/src/components/I18n.vue
@@ -9,7 +9,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 
 <script setup lang="ts" generic="T extends string | ParameterizedString">
 import { computed, h } from 'vue';
-import type { ParameterizedString } from '../../../../locales/index.js';
+import type { ParameterizedString } from 'i18n';
 
 const props = withDefaults(defineProps<{
 	src: T;
@@ -25,7 +25,7 @@ const slots = defineSlots<T extends ParameterizedString<infer R> ? { [K in R]: (
 const parsed = computed(() => {
 	let str = props.src as string;
 	const value: (string | { arg: string; })[] = [];
-	for (;;) {
+	for (; ;) {
 		const nextBracketOpen = str.indexOf('{');
 		const nextBracketClose = str.indexOf('}');
 
diff --git a/packages/frontend-embed/src/i18n.ts b/packages/frontend-embed/src/i18n.ts
index 0b2b206b7e..6a3a18df17 100644
--- a/packages/frontend-embed/src/i18n.ts
+++ b/packages/frontend-embed/src/i18n.ts
@@ -6,7 +6,7 @@
 import { markRaw } from 'vue';
 import { I18n } from '@@/js/i18n.js';
 import { locale } from '@@/js/locale.js';
-import type { Locale } from '../../../locales/index.js';
+import type { Locale } from 'i18n';
 
 export const i18n = markRaw(new I18n<Locale>(locale, _DEV_));
 
diff --git a/packages/frontend-embed/vite.config.ts b/packages/frontend-embed/vite.config.ts
index db4afb43a7..9e5c24f9d4 100644
--- a/packages/frontend-embed/vite.config.ts
+++ b/packages/frontend-embed/vite.config.ts
@@ -1,10 +1,10 @@
 import path from 'path';
 import pluginVue from '@vitejs/plugin-vue';
-import { type UserConfig, defineConfig } from 'vite';
+import { defineConfig, type UserConfig } from 'vite';
 import * as yaml from 'js-yaml';
 import { promises as fsp } from 'fs';
 
-import locales from '../../locales/index.js';
+import locales from 'i18n';
 import meta from '../../package.json';
 import packageInfo from './package.json' with { type: 'json' };
 import pluginJson5 from './vite.json5.js';
diff --git a/packages/frontend-shared/js/i18n.ts b/packages/frontend-shared/js/i18n.ts
index 3b103c4714..a42450ee88 100644
--- a/packages/frontend-shared/js/i18n.ts
+++ b/packages/frontend-shared/js/i18n.ts
@@ -3,7 +3,7 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
-import type { ILocale, ParameterizedString } from '../../../locales/index.js';
+import type { ILocale, ParameterizedString } from 'i18n';
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 type TODO = any;
diff --git a/packages/frontend-shared/js/locale.ts b/packages/frontend-shared/js/locale.ts
index 87e3922fd9..38584fbed6 100644
--- a/packages/frontend-shared/js/locale.ts
+++ b/packages/frontend-shared/js/locale.ts
@@ -4,7 +4,7 @@
  */
 
 import { lang, version } from '@@/js/config.js';
-import type { Locale } from '../../../locales/index.js';
+import type { Locale } from 'i18n';
 
 // ここはビルド時に const locale = JSON.parse("...") みたいな感じで置き換えられるので top-level await は消える
 export let locale: Locale = await window.fetch(`/assets/locales/${lang}.${version}.json`).then(r => r.json(), () => null);
diff --git a/packages/frontend-shared/js/store-boot-errors.ts b/packages/frontend-shared/js/store-boot-errors.ts
index 31e6248445..2741339165 100644
--- a/packages/frontend-shared/js/store-boot-errors.ts
+++ b/packages/frontend-shared/js/store-boot-errors.ts
@@ -3,7 +3,7 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
-import type { Locale } from '../../../locales/index.js';
+import type { Locale } from 'i18n';
 
 type BootLoaderLocaleBody = Locale['_bootErrors'] & { reload: Locale['reload'] };
 
diff --git a/packages/frontend-shared/package.json b/packages/frontend-shared/package.json
index a407df6bf8..5806414930 100644
--- a/packages/frontend-shared/package.json
+++ b/packages/frontend-shared/package.json
@@ -34,6 +34,7 @@
 		"js-built"
 	],
 	"dependencies": {
+		"i18n": "workspace:*",
 		"misskey-js": "workspace:*",
 		"vue": "3.5.24"
 	}
diff --git a/packages/frontend/.storybook/preload-locale.ts b/packages/frontend/.storybook/preload-locale.ts
index c823ff9bee..42918b44f3 100644
--- a/packages/frontend/.storybook/preload-locale.ts
+++ b/packages/frontend/.storybook/preload-locale.ts
@@ -4,7 +4,7 @@
  */
 
 import { writeFile } from 'node:fs/promises';
-import locales from '../../../locales/index.js';
+import locales from 'i18n';
 
 await writeFile(
 	new URL('locale.ts', import.meta.url),
diff --git a/packages/frontend/build.ts b/packages/frontend/build.ts
index 0401c2b9ba..0f605c745b 100644
--- a/packages/frontend/build.ts
+++ b/packages/frontend/build.ts
@@ -2,7 +2,7 @@ import * as fs from 'fs/promises';
 import url from 'node:url';
 import path from 'node:path';
 import { execa } from 'execa';
-import locales from '../../locales/index.js';
+import locales from 'i18n';
 import { LocaleInliner } from '../frontend-builder/locale-inliner.js'
 import { createLogger } from '../frontend-builder/logger';
 
diff --git a/packages/frontend/lib/vite-plugin-watch-locales.ts b/packages/frontend/lib/vite-plugin-watch-locales.ts
index 8e209d27bd..372e9039d5 100644
--- a/packages/frontend/lib/vite-plugin-watch-locales.ts
+++ b/packages/frontend/lib/vite-plugin-watch-locales.ts
@@ -4,7 +4,7 @@
  */
 
 import path from 'node:path'
-import locales from '../../../locales/index.js';
+import locales from 'i18n';
 
 const localesDir = path.resolve(__dirname, '../../../locales')
 
@@ -13,14 +13,14 @@ const localesDir = path.resolve(__dirname, '../../../locales')
  * @returns {import('vite').Plugin}
  */
 export default function pluginWatchLocales() {
-  return {
-    name: 'watch-locales',
+	return {
+		name: 'watch-locales',
 
-    configureServer(server) {
-      const localeYmlPaths = Object.keys(locales).map(locale => path.join(localesDir, `${locale}.yml`));
+		configureServer(server) {
+			const localeYmlPaths = Object.keys(locales).map(locale => path.join(localesDir, `${locale}.yml`));
 
-      // watcherにパスを追加
-      server.watcher.add(localeYmlPaths);
+			// watcherにパスを追加
+			server.watcher.add(localeYmlPaths);
 
 			server.watcher.on('change', (filePath) => {
 				if (localeYmlPaths.includes(filePath)) {
@@ -31,6 +31,6 @@ export default function pluginWatchLocales() {
 					})
 				}
 			});
-    },
-  };
+		},
+	};
 }
diff --git a/packages/frontend/package.json b/packages/frontend/package.json
index ec7b39312a..c3baf5ea74 100644
--- a/packages/frontend/package.json
+++ b/packages/frontend/package.json
@@ -20,6 +20,7 @@
 		"@discordapp/twemoji": "16.0.1",
 		"@github/webauthn-json": "2.1.1",
 		"@mcaptcha/vanilla-glue": "0.1.0-alpha-3",
+		"i18n": "workspace:*",
 		"@misskey-dev/browser-image-resizer": "2024.1.0",
 		"@rollup/plugin-json": "6.1.0",
 		"@rollup/plugin-replace": "6.0.3",
diff --git a/packages/frontend/src/components/global/I18n.vue b/packages/frontend/src/components/global/I18n.vue
index 6b7723e6ac..9866e50958 100644
--- a/packages/frontend/src/components/global/I18n.vue
+++ b/packages/frontend/src/components/global/I18n.vue
@@ -9,7 +9,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 
 <script setup lang="ts" generic="T extends string | ParameterizedString">
 import { computed, h } from 'vue';
-import type { ParameterizedString } from '../../../../../locales/index.js';
+import type { ParameterizedString } from 'i18n';
 
 const props = withDefaults(defineProps<{
 	src: T;
@@ -25,7 +25,7 @@ const slots = defineSlots<T extends ParameterizedString<infer R> ? { [K in R]: (
 const parsed = computed(() => {
 	let str = props.src as string;
 	const value: (string | { arg: string; })[] = [];
-	for (;;) {
+	for (; ;) {
 		const nextBracketOpen = str.indexOf('{');
 		const nextBracketClose = str.indexOf('}');
 
diff --git a/packages/frontend/src/i18n.ts b/packages/frontend/src/i18n.ts
index 0b2b206b7e..6a3a18df17 100644
--- a/packages/frontend/src/i18n.ts
+++ b/packages/frontend/src/i18n.ts
@@ -6,7 +6,7 @@
 import { markRaw } from 'vue';
 import { I18n } from '@@/js/i18n.js';
 import { locale } from '@@/js/locale.js';
-import type { Locale } from '../../../locales/index.js';
+import type { Locale } from 'i18n';
 
 export const i18n = markRaw(new I18n<Locale>(locale, _DEV_));
 
diff --git a/packages/frontend/test/i18n.test.ts b/packages/frontend/test/i18n.test.ts
index a51dfc6c4e..14b89f898d 100644
--- a/packages/frontend/test/i18n.test.ts
+++ b/packages/frontend/test/i18n.test.ts
@@ -5,7 +5,7 @@
 
 import { describe, expect, it } from 'vitest';
 import { I18n } from '../../frontend-shared/js/i18n.js'; // @@で参照できなかったので
-import type { ParameterizedString } from '../../../locales/index.js';
+import type { ParameterizedString } from 'i18n';
 
 // TODO: このテストはfrontend-sharedに移動する
 
diff --git a/packages/frontend/test/init.ts b/packages/frontend/test/init.ts
index e38338cf95..28848f6c2f 100644
--- a/packages/frontend/test/init.ts
+++ b/packages/frontend/test/init.ts
@@ -7,13 +7,13 @@ import { vi } from 'vitest';
 import createFetchMock from 'vitest-fetch-mock';
 import type { Ref } from 'vue';
 import { ref } from 'vue';
+// Set i18n
+import locales from 'i18n';
+import { updateI18n } from '@/i18n.js';
 
 const fetchMocker = createFetchMock(vi);
 fetchMocker.enableMocks();
 
-// Set i18n
-import locales from '../../../locales/index.js';
-import { updateI18n } from '@/i18n.js';
 updateI18n(locales['en-US']);
 
 // XXX: misskey-js panics if WebSocket is not defined
diff --git a/packages/frontend/vite.config.ts b/packages/frontend/vite.config.ts
index 6f320e99c9..c9c20b23ea 100644
--- a/packages/frontend/vite.config.ts
+++ b/packages/frontend/vite.config.ts
@@ -2,18 +2,18 @@ import path from 'path';
 import pluginReplace from '@rollup/plugin-replace';
 import pluginVue from '@vitejs/plugin-vue';
 import pluginGlsl from 'vite-plugin-glsl';
-import { defineConfig } from 'vite';
 import type { UserConfig } from 'vite';
+import { defineConfig } from 'vite';
 import * as yaml from 'js-yaml';
 import { promises as fsp } from 'fs';
 
-import locales from '../../locales/index.js';
+import locales from 'i18n';
 import meta from '../../package.json';
 import packageInfo from './package.json' with { type: 'json' };
 import pluginUnwindCssModuleClassName from './lib/rollup-plugin-unwind-css-module-class-name.js';
 import pluginJson5 from './vite.json5.js';
-import pluginCreateSearchIndex from './lib/vite-plugin-create-search-index.js';
 import type { Options as SearchIndexOptions } from './lib/vite-plugin-create-search-index.js';
+import pluginCreateSearchIndex from './lib/vite-plugin-create-search-index.js';
 import pluginWatchLocales from './lib/vite-plugin-watch-locales.js';
 import { pluginRemoveUnrefI18n } from '../frontend-builder/rollup-plugin-remove-unref-i18n.js';
 
diff --git a/packages/i18n/README.md b/packages/i18n/README.md
new file mode 100644
index 0000000000..1013b84ea6
--- /dev/null
+++ b/packages/i18n/README.md
@@ -0,0 +1,5 @@
+# Misskey i18n
+
+Misskey の言語ファイル本体 (ja-JP.yml など) はリポジトリ直下の `/locales` に置かれており、そこから Crowdin 連携やビルド資産が生成されます。
+
+このパッケージは Misskey モノレポ内で、これらの言語ファイルを共通で扱うためのヘルパー群や型情報をまとめる位置づけです。バックエンド / フロントエンド / Service Worker など各パッケージが同じ翻訳データと型定義を利用できるようにすることを目的としており、npm での外部配布は想定していません。
diff --git a/packages/i18n/build.ts b/packages/i18n/build.ts
new file mode 100644
index 0000000000..a6bbf7dc63
--- /dev/null
+++ b/packages/i18n/build.ts
@@ -0,0 +1,163 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import fs from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { dirname, resolve } from 'node:path';
+import { watch as chokidarWatch } from 'chokidar';
+import * as esbuild from 'esbuild';
+import { build } from 'esbuild';
+import { execa } from 'execa';
+import { globSync } from 'glob';
+import { generateLocaleInterface } from './scripts/generateLocaleInterface.js';
+import type { BuildOptions, BuildResult, Plugin, PluginBuild } from 'esbuild';
+
+const _filename = fileURLToPath(import.meta.url);
+const _dirname = dirname(_filename);
+const _package = JSON.parse(fs.readFileSync(_dirname + '/package.json', 'utf-8'));
+const _rootPackageDir = resolve(_dirname, '../../');
+const _rootPackage = JSON.parse(fs.readFileSync(resolve(_rootPackageDir, 'package.json'), 'utf-8'));
+const _frontendLocalesDir = resolve(_dirname, '../../built/_frontend_dist_/locales');
+const _localesDir = resolve(_rootPackageDir, 'locales');
+
+const entryPoints = globSync('./src/**/**.{ts,tsx}');
+
+const options: BuildOptions = {
+	entryPoints,
+	minify: process.env.NODE_ENV === 'production',
+	sourceRoot: 'src',
+	outdir: './built',
+	target: 'es2022',
+	platform: 'node',
+	format: 'esm',
+	sourcemap: 'linked',
+};
+
+// コマンドライン引数を取得
+const args = process.argv.slice(2).map(arg => arg.toLowerCase());
+
+// built配下をすべて削除する
+if (!args.includes('--no-clean')) {
+	fs.rmSync('./built', { recursive: true, force: true });
+}
+
+if (args.includes('--watch')) {
+	await watchSrc();
+} else {
+	await buildSrc();
+}
+
+function copyLocales(): void {
+	const srcDir = _localesDir;
+	const destDir = resolve(_dirname, 'built/locales');
+
+	fs.mkdirSync(destDir, { recursive: true });
+
+	const files = fs.readdirSync(srcDir).filter(f => f.endsWith('.yml'));
+	for (const file of files) {
+		fs.copyFileSync(resolve(srcDir, file), resolve(destDir, file));
+	}
+	console.log(`[${_package.name}] locales copied (${files.length} files).`);
+}
+
+/**
+ * フロントエンド用の locale JSON を書き出す
+ * Service Worker が HTTP 経由で取得するために必要
+ */
+async function writeFrontendLocalesJson(): Promise<void> {
+	// 動的 import でビルド済みモジュールから読み込み（循環参照回避）
+	const { writeFrontendLocalesJson: write } = await import('./built/index.js');
+	await write(_frontendLocalesDir, _rootPackage.version);
+	console.log(`[${_package.name}] frontend locales JSON written to ${_frontendLocalesDir}`);
+}
+
+async function buildSrc(): Promise<void> {
+	console.log(`[${_package.name}] start building...`);
+
+	await generateLocaleInterface(_localesDir);
+
+	await build(options)
+		.then(() => {
+			console.log(`[${_package.name}] build succeeded.`);
+		})
+		.catch((err) => {
+			process.stderr.write(err.stderr);
+			process.exit(1);
+		});
+
+	copyLocales();
+	await writeFrontendLocalesJson();
+
+	if (process.env.NODE_ENV === 'production') {
+		console.log(`[${_package.name}] skip building d.ts because NODE_ENV is production.`);
+	} else {
+		await buildDts();
+	}
+
+	console.log(`[${_package.name}] finish building.`);
+}
+
+function buildDts(): Promise<unknown> {
+	return execa(
+		'tsc',
+		[
+			'--project', 'tsconfig.json',
+			'--rootDir', 'src',
+			'--outDir', 'built',
+			'--declaration', 'true',
+			'--emitDeclarationOnly', 'true',
+		],
+		{
+			stdout: process.stdout,
+			stderr: process.stderr,
+		},
+	);
+}
+
+async function watchSrc(): Promise<void> {
+	const localesWatcher = chokidarWatch(_localesDir, {
+		ignoreInitial: true,
+	});
+	localesWatcher.on('all', async (event, path) => {
+		if (!path.endsWith('.yml')) return;
+		console.log(`[${_package.name}] locales changed: ${event} ${path}`);
+		copyLocales();
+		await writeFrontendLocalesJson();
+		await generateLocaleInterface(_localesDir);
+	});
+
+	const plugins: Plugin[] = [{
+		name: 'gen-dts',
+		setup(build: PluginBuild) {
+			build.onStart(() => {
+				console.log(`[${_package.name}] detect changed...`);
+			});
+			build.onEnd(async (result: BuildResult) => {
+				if (result.errors.length > 0) {
+					console.error(`[${_package.name}] watch build failed:`, result);
+					return;
+				}
+				await buildDts();
+			});
+		},
+	}];
+
+	console.log(`[${_package.name}] start watching...`);
+
+	const context = await esbuild.context({ ...options, plugins });
+	await context.watch();
+
+	await new Promise((resolve, reject) => {
+		process.on('SIGHUP', resolve);
+		process.on('SIGINT', resolve);
+		process.on('SIGTERM', resolve);
+		process.on('uncaughtException', reject);
+		process.on('exit', resolve);
+	}).finally(async () => {
+		await context.dispose();
+		await localesWatcher.close();
+		console.log(`[${_package.name}] finish watching.`);
+	});
+}
diff --git a/packages/i18n/eslint.config.js b/packages/i18n/eslint.config.js
new file mode 100644
index 0000000000..f18bc4a4be
--- /dev/null
+++ b/packages/i18n/eslint.config.js
@@ -0,0 +1,35 @@
+import tsParser from '@typescript-eslint/parser';
+import sharedConfig from '../shared/eslint.config.js';
+
+// eslint-disable-next-line import/no-default-export
+export default [
+	...sharedConfig,
+	{
+		ignores: [
+			'**/node_modules',
+			'built',
+			'coverage',
+			'vitest.config.ts',
+			'test',
+			'test-d',
+			'generator',
+		],
+	},
+	{
+		files: ['**/*.ts', '**/*.tsx'],
+		languageOptions: {
+			parserOptions: {
+				parser: tsParser,
+				project: ['./tsconfig.eslint.json'],
+				sourceType: 'module',
+				tsconfigRootDir: import.meta.dirname,
+			},
+		},
+	},
+	{
+		files: ['src/autogen/**/*.ts', 'src/autogen/**/*.tsx'],
+		rules: {
+			'@stylistic/indent': 'off',
+		},
+	},
+];
diff --git a/packages/i18n/package.json b/packages/i18n/package.json
new file mode 100644
index 0000000000..d06e485da0
--- /dev/null
+++ b/packages/i18n/package.json
@@ -0,0 +1,42 @@
+{
+	"name": "i18n",
+	"type": "module",
+	"private": true,
+	"main": "./built/index.js",
+	"types": "./built/index.d.ts",
+	"exports": {
+		".": {
+			"types": "./built/index.d.ts",
+			"import": "./built/index.js"
+		}
+	},
+	"scripts": {
+		"generate": "tsx scripts/generateLocaleInterface.ts",
+		"verify": "tsx scripts/verify.ts",
+		"build": "tsx ./build.ts",
+		"watch": "nodemon -w package.json -e json --exec \"tsx ./build.ts --watch\"",
+		"tsd": "tsd",
+		"typecheck": "tsc --noEmit",
+		"lint": "pnpm typecheck && pnpm eslint",
+		"lint:fix": "pnpm eslint --fix"
+	},
+	"files": [
+		"built"
+	],
+	"devDependencies": {
+		"@types/js-yaml": "4.0.9",
+		"@types/node": "24.10.1",
+		"@typescript-eslint/eslint-plugin": "8.47.0",
+		"@typescript-eslint/parser": "8.47.0",
+		"chokidar": "4.0.3",
+		"esbuild": "0.27.0",
+		"execa": "9.6.0",
+		"glob": "11.1.0",
+		"nodemon": "3.1.11",
+		"tsx": "4.20.6",
+		"typescript": "5.9.3"
+	},
+	"dependencies": {
+		"js-yaml": "4.1.1"
+	}
+}
diff --git a/packages/i18n/scripts/generateLocaleInterface.ts b/packages/i18n/scripts/generateLocaleInterface.ts
new file mode 100644
index 0000000000..1c0f5c6a79
--- /dev/null
+++ b/packages/i18n/scripts/generateLocaleInterface.ts
@@ -0,0 +1,153 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import * as fs from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { dirname, resolve } from 'node:path';
+import * as yaml from 'js-yaml';
+import ts from 'typescript';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+const parameterRegExp = /\{(\w+)\}/g;
+
+interface LocaleRecord {
+	[key: string]: string | LocaleRecord;
+}
+
+function createMemberType(item: string | LocaleRecord): ts.TypeNode {
+	if (typeof item !== 'string') {
+		return ts.factory.createTypeLiteralNode(createMembers(item));
+	}
+	const parameters = Array.from(
+		item.matchAll(parameterRegExp),
+		([, parameter]) => parameter,
+	);
+	return parameters.length
+		? ts.factory.createTypeReferenceNode(
+			ts.factory.createIdentifier('ParameterizedString'),
+			[
+				ts.factory.createUnionTypeNode(
+					parameters.map((parameter) =>
+						ts.factory.createLiteralTypeNode(ts.factory.createStringLiteral(parameter)),
+					),
+				),
+			],
+		)
+		: ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword);
+}
+
+function createMembers(record: LocaleRecord): ts.TypeElement[] {
+	return Object.entries(record).map(([k, v]) => {
+		const node = ts.factory.createPropertySignature(
+			undefined,
+			ts.factory.createStringLiteral(k),
+			undefined,
+			createMemberType(v),
+		);
+		if (typeof v === 'string') {
+			ts.addSyntheticLeadingComment(
+				node,
+				ts.SyntaxKind.MultiLineCommentTrivia,
+				`*
+ * ${v.replace(/\n/g, '\n * ')}
+ `,
+				true,
+			);
+		}
+		return node;
+	});
+}
+
+export async function generateLocaleInterface(localesDir: string): Promise<void> {
+	const locale = yaml.load(fs.readFileSync(`${localesDir}/ja-JP.yml`, 'utf-8').toString()) as LocaleRecord;
+	const members = createMembers(locale);
+
+	const elements: ts.Statement[] = [
+		ts.factory.createImportDeclaration(
+			undefined,
+			ts.factory.createImportClause(
+				false,
+				undefined,
+				ts.factory.createNamedImports([
+					ts.factory.createImportSpecifier(
+						true,
+						undefined,
+						ts.factory.createIdentifier('ILocale'),
+					),
+					ts.factory.createImportSpecifier(
+						true,
+						undefined,
+						ts.factory.createIdentifier('ParameterizedString'),
+					),
+				]),
+			),
+			ts.factory.createStringLiteral('../types.js'),
+			undefined,
+		),
+		ts.factory.createInterfaceDeclaration(
+			[ts.factory.createToken(ts.SyntaxKind.ExportKeyword)],
+			ts.factory.createIdentifier('Locale'),
+			undefined,
+			[
+				ts.factory.createHeritageClause(ts.SyntaxKind.ExtendsKeyword, [
+					ts.factory.createExpressionWithTypeArguments(
+						ts.factory.createIdentifier('ILocale'),
+						undefined,
+					),
+				]),
+			],
+			members,
+		),
+	];
+
+	ts.addSyntheticLeadingComment(
+		elements[0],
+		ts.SyntaxKind.MultiLineCommentTrivia,
+		' eslint-disable ',
+		true,
+	);
+	ts.addSyntheticLeadingComment(
+		elements[0],
+		ts.SyntaxKind.SingleLineCommentTrivia,
+		' This file is generated by scripts/generateLocaleInterface.ts',
+		true,
+	);
+	ts.addSyntheticLeadingComment(
+		elements[0],
+		ts.SyntaxKind.SingleLineCommentTrivia,
+		' Do not edit this file directly.',
+		true,
+	);
+
+	const printed = ts
+		.createPrinter({
+			newLine: ts.NewLineKind.LineFeed,
+		})
+		.printList(
+			ts.ListFormat.MultiLine,
+			ts.factory.createNodeArray(elements),
+			ts.createSourceFile(
+				'locale.ts',
+				'',
+				ts.ScriptTarget.ESNext,
+				true,
+				ts.ScriptKind.TS,
+			),
+		);
+
+	const autogenDir = `${__dirname}/../src/autogen`;
+	fs.mkdirSync(autogenDir, { recursive: true });
+
+	// 一瞬ファイルが存在しなくなって途切れる→不安定になるらしいので、リネームで対処
+	fs.writeFileSync(`${autogenDir}/_locale.ts`, printed, 'utf-8');
+	fs.renameSync(`${autogenDir}/_locale.ts`, `${autogenDir}/locale.ts`);
+}
+
+// スクリプトとして直接実行された場合
+const isMain = import.meta.url === `file://${process.argv[1]}`;
+if (isMain) {
+	await generateLocaleInterface(resolve(__dirname, '../../../locales'));
+}
diff --git a/packages/i18n/scripts/verify.ts b/packages/i18n/scripts/verify.ts
new file mode 100644
index 0000000000..877966e49f
--- /dev/null
+++ b/packages/i18n/scripts/verify.ts
@@ -0,0 +1,70 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+let valid = true;
+
+interface LocaleRecord {
+	[key: string]: string | LocaleRecord;
+}
+
+interface ErrorData {
+	expected?: string;
+	actual?: string;
+	parameter?: string;
+}
+
+function writeError(type: string, lang: string, tree: string, data: ErrorData): void {
+	process.stderr.write(JSON.stringify({ type, lang, tree, data }));
+	process.stderr.write('\n');
+	valid = false;
+}
+
+function verify(expected: LocaleRecord, actual: LocaleRecord, lang: string, trace?: string): void {
+	for (const key in expected) {
+		if (!Object.prototype.hasOwnProperty.call(actual, key)) {
+			continue;
+		}
+		if (typeof expected[key] === 'object') {
+			if (typeof actual[key] !== 'object') {
+				writeError('mismatched_type', lang, trace ? `${trace}.${key}` : key, { expected: 'object', actual: typeof actual[key] });
+				continue;
+			}
+			verify(expected[key] as LocaleRecord, actual[key] as LocaleRecord, lang, trace ? `${trace}.${key}` : key);
+		} else if (typeof expected[key] === 'string') {
+			switch (typeof actual[key]) {
+				case 'object':
+					writeError('mismatched_type', lang, trace ? `${trace}.${key}` : key, { expected: 'string', actual: 'object' });
+					break;
+				case 'undefined':
+					continue;
+				case 'string': {
+					const expectedParameters = new Set((expected[key] as string).match(/\{[^}]+\}/g)?.map((s) => s.slice(1, -1)));
+					const actualParameters = new Set((actual[key] as string).match(/\{[^}]+\}/g)?.map((s) => s.slice(1, -1)));
+					for (const parameter of expectedParameters) {
+						if (!actualParameters.has(parameter)) {
+							writeError('missing_parameter', lang, trace ? `${trace}.${key}` : key, { parameter });
+						}
+					}
+				}
+			}
+		}
+	}
+}
+
+// index.tsはtsのまま動かすことを想定していない（ビルド成果物を外部に公開する）.
+// よってビルド後のものを検証する
+const locales = await import('../built/index.js');
+const { 'ja-JP': original, ...verifiees } = locales as unknown as Record<string, LocaleRecord>;
+
+for (const lang in verifiees) {
+	if (!Object.prototype.hasOwnProperty.call(locales, lang)) {
+		continue;
+	}
+	verify(original, verifiees[lang], lang);
+}
+
+if (!valid) {
+	process.exit(1);
+}
diff --git a/locales/index.d.ts b/packages/i18n/src/autogen/locale.ts
index 7f0efd22e8..8f94aab555 100644
--- a/locales/index.d.ts
+++ b/packages/i18n/src/autogen/locale.ts
@@ -1,13 +1,7 @@
 /* eslint-disable */
-// This file is generated by locales/generateDTS.js
+// This file is generated by scripts/generateLocaleInterface.ts
 // Do not edit this file directly.
-declare const kParameters: unique symbol;
-export type ParameterizedString<T extends string = string> = string & {
-    [kParameters]: T;
-};
-export interface ILocale {
-    [_: string]: string | ParameterizedString | ILocale;
-}
+import { type ILocale, type ParameterizedString } from "../types.js";
 export interface Locale extends ILocale {
     /**
      * 日本語
@@ -13089,8 +13083,3 @@ export interface Locale extends ILocale {
         "mfm": string;
     };
 }
-declare const locales: {
-    [lang: string]: Locale;
-};
-export function build(): Locale;
-export default locales;
diff --git a/packages/i18n/src/index.ts b/packages/i18n/src/index.ts
new file mode 100644
index 0000000000..e428267748
--- /dev/null
+++ b/packages/i18n/src/index.ts
@@ -0,0 +1,166 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+/**
+ * Languages Loader
+ */
+
+import * as fs from 'node:fs';
+import * as yaml from 'js-yaml';
+import type { Locale } from './autogen/locale.js';
+import type { ILocale, ParameterizedString } from './types.js';
+
+const languages = [
+	'ar-SA',
+	'ca-ES',
+	'cs-CZ',
+	'da-DK',
+	'de-DE',
+	'en-US',
+	'es-ES',
+	'fr-FR',
+	'id-ID',
+	'it-IT',
+	'ja-JP',
+	'ja-KS',
+	'kab-KAB',
+	'kn-IN',
+	'ko-KR',
+	'nl-NL',
+	'no-NO',
+	'pl-PL',
+	'pt-PT',
+	'ru-RU',
+	'sk-SK',
+	'th-TH',
+	'tr-TR',
+	'ug-CN',
+	'uk-UA',
+	'vi-VN',
+	'zh-CN',
+	'zh-TW',
+] as const;
+
+type Language = typeof languages[number];
+
+const primaries = {
+	'en': 'US',
+	'ja': 'JP',
+	'zh': 'CN',
+} as const satisfies Record<string, string>;
+
+type PrimaryLang = keyof typeof primaries;
+
+type Locales = Record<Language, ILocale>;
+
+/**
+ * オブジェクトを再帰的にマージする
+ */
+function merge<T extends ILocale>(...args: (T | ILocale | undefined)[]): T {
+	return args.reduce<ILocale>((a, c) => ({
+		...a,
+		...c,
+		...Object.entries(a)
+			.filter(([k]) => c && typeof c[k] === 'object')
+			.reduce<Record<string, ILocale[string]>>((acc, [k, v]) => {
+				acc[k] = merge(v as ILocale, (c as ILocale)[k] as ILocale);
+				return acc;
+			}, {}),
+	}), {} as ILocale) as T;
+}
+
+/**
+ * 何故か文字列にバックスペース文字が混入することがあり、YAMLが壊れるので取り除く
+ */
+function clean (text: string) {
+	return text.replace(new RegExp(String.fromCodePoint(0x08), 'g'), '');
+}
+
+/**
+ * 空文字列が入ることがあり、フォールバックが動作しなくなるのでプロパティごと消す
+ */
+function removeEmpty<T extends ILocale>(obj: T): T {
+	for (const [k, v] of Object.entries(obj)) {
+		if (v === '') {
+			delete obj[k];
+		} else if (typeof v === 'object') {
+			removeEmpty(v as ILocale);
+		}
+	}
+	return obj;
+}
+
+function build(): Record<Language, Locale> {
+	// vitestの挙動を調整するため、一度ローカル変数化する必要がある
+	// https://github.com/vitest-dev/vitest/issues/3988#issuecomment-1686599577
+	// https://github.com/misskey-dev/misskey/pull/14057#issuecomment-2192833785
+	const metaUrl = import.meta.url;
+	const locales = languages.reduce<Locales>((a, lang) => {
+		a[lang] = (yaml.load(clean(fs.readFileSync(new URL(`./locales/${lang}.yml`, metaUrl), 'utf-8'))) ?? {}) as ILocale;
+		return a;
+	}, {} as Locales);
+
+	removeEmpty(locales);
+
+	return Object.entries(locales).reduce<Record<Language, Locale>>((a, [k, v]) => {
+		const lang = k.split('-')[0];
+		const key = k as Language;
+
+		switch (key) {
+			case 'ja-JP':
+				a[key] = v as Locale;
+				break;
+			case 'ja-KS':
+			case 'en-US':
+				a[key] = merge<Locale>(locales['ja-JP'] as Locale, v);
+				break;
+			default: {
+				const primaryLang = lang as PrimaryLang;
+				const primaryKey = (lang in primaries ? `${lang}-${primaries[primaryLang]}` : undefined) as Language | undefined;
+				a[key] = merge<Locale>(
+					locales['ja-JP'] as Locale,
+					locales['en-US'],
+					primaryKey ? locales[primaryKey] : {},
+					v,
+				);
+				break;
+			}
+		}
+
+		return a;
+	}, {} as Record<Language, Locale>);
+}
+
+const locales = build() as {
+	[lang: string]: Locale;
+};
+
+/**
+ * フロントエンド用の locale JSON を書き出す
+ * Service Worker が HTTP 経由で取得するために必要
+ * @param destDir 出力先ディレクトリ（例: built/_frontend_dist_/locales）
+ * @param version バージョン文字列（ファイル名とJSON内に埋め込まれる）
+ */
+async function writeFrontendLocalesJson(destDir: string, version: string): Promise<void> {
+	const { mkdir, writeFile } = await import('node:fs/promises');
+	const { resolve } = await import('node:path');
+
+	await mkdir(destDir, { recursive: true });
+
+	const builtLocales = build();
+	const v = { '_version_': version };
+
+	for (const [lang, locale] of Object.entries(builtLocales)) {
+		await writeFile(
+			resolve(destDir, `${lang}.${version}.json`),
+			JSON.stringify({ ...locale, ...v }),
+			'utf-8',
+		);
+	}
+}
+
+export { locales, build, writeFrontendLocalesJson };
+export type { Language, Locale, ILocale, ParameterizedString };
+export default locales;
diff --git a/packages/i18n/src/types.ts b/packages/i18n/src/types.ts
new file mode 100644
index 0000000000..cf9568e792
--- /dev/null
+++ b/packages/i18n/src/types.ts
@@ -0,0 +1,14 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+declare const kParameters: unique symbol;
+
+export type ParameterizedString<T extends string = string> = string & {
+	[kParameters]: T;
+};
+
+export interface ILocale {
+	[_: string]: string | ParameterizedString | ILocale;
+}
diff --git a/packages/i18n/tsconfig.eslint.json b/packages/i18n/tsconfig.eslint.json
new file mode 100644
index 0000000000..5aea206cd1
--- /dev/null
+++ b/packages/i18n/tsconfig.eslint.json
@@ -0,0 +1,8 @@
+{
+    "extends": "./tsconfig.json",
+    "include": [
+        "src/**/*.ts",
+        "scripts/**/*.ts",
+        "build.ts"
+    ]
+}
\ No newline at end of file
diff --git a/packages/i18n/tsconfig.json b/packages/i18n/tsconfig.json
new file mode 100644
index 0000000000..31cd39b8ba
--- /dev/null
+++ b/packages/i18n/tsconfig.json
@@ -0,0 +1,14 @@
+{
+	"compilerOptions": {
+		"target": "ESNext",
+		"module": "NodeNext",
+		"moduleResolution": "NodeNext",
+		"esModuleInterop": true,
+		"forceConsistentCasingInFileNames": true,
+		"strict": true,
+		"skipLibCheck": true
+	},
+	"include": [
+		"src/**/*.ts"
+	]
+}
\ No newline at end of file
diff --git a/packages/sw/build.js b/packages/sw/build.js
index a9c2e428c0..d5ca028da5 100644
--- a/packages/sw/build.js
+++ b/packages/sw/build.js
@@ -7,8 +7,9 @@
 
 import { fileURLToPath } from 'node:url';
 import * as esbuild from 'esbuild';
-import locales from '../../locales/index.js';
+import locales from 'i18n';
 import meta from '../../package.json' with { type: 'json' };
+
 const watch = process.argv[2]?.includes('watch');
 
 const __dirname = fileURLToPath(new URL('.', import.meta.url));
diff --git a/packages/sw/package.json b/packages/sw/package.json
index 51d78511c2..0fe10256ce 100644
--- a/packages/sw/package.json
+++ b/packages/sw/package.json
@@ -9,6 +9,7 @@
 		"lint": "pnpm typecheck && pnpm eslint"
 	},
 	"dependencies": {
+		"i18n": "workspace:*",
 		"esbuild": "0.27.0",
 		"idb-keyval": "6.2.2",
 		"misskey-js": "workspace:*"
diff --git a/packages/sw/src/scripts/lang.ts b/packages/sw/src/scripts/lang.ts
index 3000160e41..40b6aa4e7b 100644
--- a/packages/sw/src/scripts/lang.ts
+++ b/packages/sw/src/scripts/lang.ts
@@ -8,7 +8,7 @@
  */
 import { get, set } from 'idb-keyval';
 import { I18n } from '@@/js/i18n.js';
-import type { Locale } from '../../../../locales/index.js';
+import type { Locale } from 'i18n';
 
 class SwLang {
 	public cacheName = `mk-cache-${_VERSION_}`;
diff --git a/packages/sw/src/sw.ts b/packages/sw/src/sw.ts
index 298af4b4b6..5cece73401 100644
--- a/packages/sw/src/sw.ts
+++ b/packages/sw/src/sw.ts
@@ -7,7 +7,7 @@ import { get } from 'idb-keyval';
 import * as Misskey from 'misskey-js';
 import type { PushNotificationDataMap } from '@/types.js';
 import type { I18n } from '@@/js/i18n.js';
-import type { Locale } from '../../../locales/index.js';
+import type { Locale } from 'i18n';
 import { createEmptyNotification, createNotification } from '@/scripts/create-notification.js';
 import { swLang } from '@/scripts/lang.js';
 import * as swos from '@/scripts/operations.js';
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 3793bb79c1..02a6124d7d 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -77,6 +77,9 @@ importers:
       globals:
         specifier: 16.5.0
         version: 16.5.0
+      i18n:
+        specifier: workspace:*
+        version: link:packages/i18n
       ncp:
         specifier: 2.0.0
         version: 2.0.0
@@ -785,6 +788,9 @@ importers:
       frontend-shared:
         specifier: workspace:*
         version: link:../frontend-shared
+      i18n:
+        specifier: workspace:*
+        version: link:../i18n
       icons-subsetter:
         specifier: workspace:*
         version: link:../icons-subsetter
@@ -1086,6 +1092,9 @@ importers:
       estree-walker:
         specifier: 3.0.3
         version: 3.0.3
+      i18n:
+        specifier: workspace:*
+        version: link:../i18n
       magic-string:
         specifier: 0.30.21
         version: 0.30.21
@@ -1147,6 +1156,9 @@ importers:
       frontend-shared:
         specifier: workspace:*
         version: link:../frontend-shared
+      i18n:
+        specifier: workspace:*
+        version: link:../i18n
       icons-subsetter:
         specifier: workspace:*
         version: link:../icons-subsetter
@@ -1286,6 +1298,9 @@ importers:
 
   packages/frontend-shared:
     dependencies:
+      i18n:
+        specifier: workspace:*
+        version: link:../i18n
       misskey-js:
         specifier: workspace:*
         version: link:../misskey-js
@@ -1318,6 +1333,46 @@ importers:
         specifier: 10.2.0
         version: 10.2.0(eslint@9.39.1)
 
+  packages/i18n:
+    dependencies:
+      js-yaml:
+        specifier: 4.1.1
+        version: 4.1.1
+    devDependencies:
+      '@types/js-yaml':
+        specifier: 4.0.9
+        version: 4.0.9
+      '@types/node':
+        specifier: 24.10.1
+        version: 24.10.1
+      '@typescript-eslint/eslint-plugin':
+        specifier: 8.47.0
+        version: 8.47.0(@typescript-eslint/parser@8.47.0(eslint@9.39.1)(typescript@5.9.3))(eslint@9.39.1)(typescript@5.9.3)
+      '@typescript-eslint/parser':
+        specifier: 8.47.0
+        version: 8.47.0(eslint@9.39.1)(typescript@5.9.3)
+      chokidar:
+        specifier: 4.0.3
+        version: 4.0.3
+      esbuild:
+        specifier: 0.27.0
+        version: 0.27.0
+      execa:
+        specifier: 9.6.0
+        version: 9.6.0
+      glob:
+        specifier: 11.1.0
+        version: 11.1.0
+      nodemon:
+        specifier: 3.1.11
+        version: 3.1.11
+      tsx:
+        specifier: 4.20.6
+        version: 4.20.6
+      typescript:
+        specifier: 5.9.3
+        version: 5.9.3
+
   packages/icons-subsetter:
     dependencies:
       '@tabler/icons-webfont':
@@ -1519,6 +1574,9 @@ importers:
       esbuild:
         specifier: 0.27.0
         version: 0.27.0
+      i18n:
+        specifier: workspace:*
+        version: link:../i18n
       idb-keyval:
         specifier: 6.2.2
         version: 6.2.2
diff --git a/pnpm-workspace.yaml b/pnpm-workspace.yaml
index c5607f7533..9d43518ff6 100644
--- a/pnpm-workspace.yaml
+++ b/pnpm-workspace.yaml
@@ -4,6 +4,7 @@ packages:
   - packages/frontend
   - packages/frontend-builder
   - packages/frontend-embed
+  - packages/i18n
   - packages/icons-subsetter
   - packages/sw
   - packages/misskey-js
diff --git a/scripts/build-assets.mjs b/scripts/build-assets.mjs
index e610a72380..34883e3513 100644
--- a/scripts/build-assets.mjs
+++ b/scripts/build-assets.mjs
@@ -11,9 +11,7 @@ import * as yaml from 'js-yaml';
 import postcss from 'postcss';
 import * as terser from 'terser';
 
-import { build as buildLocales } from '../locales/index.js';
-import generateDTS from '../locales/generateDTS.js';
-import meta from '../package.json' with { type: "json" };
+import { locales } from 'i18n';
 import buildTarball from './tarball.mjs';
 
 const configDir = fileURLToPath(new URL('../.config', import.meta.url));
@@ -23,86 +21,59 @@ const configPath = process.env.MISSKEY_CONFIG_YML
 		? path.resolve(configDir, 'test.yml')
 		: path.resolve(configDir, 'default.yml');
 
-let locales = buildLocales();
-
 async function loadConfig() {
 	return fs.readFile(configPath, 'utf-8').then(data => yaml.load(data)).catch(() => null);
 }
 
 async function copyFrontendFonts() {
-  await fs.cp('./packages/frontend/node_modules/three/examples/fonts', './built/_frontend_dist_/fonts', { dereference: true, recursive: true });
-}
-
-async function copyFrontendLocales() {
-  generateDTS();
-
-  await fs.mkdir('./built/_frontend_dist_/locales', { recursive: true });
-
-  const v = { '_version_': meta.version };
-
-  for (const [lang, locale] of Object.entries(locales)) {
-    await fs.writeFile(`./built/_frontend_dist_/locales/${lang}.${meta.version}.json`, JSON.stringify({ ...locale, ...v }), 'utf-8');
-  }
+	await fs.cp('./packages/frontend/node_modules/three/examples/fonts', './built/_frontend_dist_/fonts', { dereference: true, recursive: true });
 }
 
 async function copyBackendViews() {
-  await fs.cp('./packages/backend/src/server/web/views', './packages/backend/built/server/web/views', { recursive: true });
+	await fs.cp('./packages/backend/src/server/web/views', './packages/backend/built/server/web/views', { recursive: true });
 }
 
 async function buildBackendScript() {
-  await fs.mkdir('./packages/backend/built/server/web', { recursive: true });
+	await fs.mkdir('./packages/backend/built/server/web', { recursive: true });
 
-  for (const file of [
-    './packages/backend/src/server/web/boot.js',
-    './packages/backend/src/server/web/boot.embed.js',
-    './packages/backend/src/server/web/bios.js',
-    './packages/backend/src/server/web/cli.js',
-    './packages/backend/src/server/web/error.js',
-  ]) {
-    let source = await fs.readFile(file, { encoding: 'utf-8' });
-    source = source.replaceAll('LANGS', JSON.stringify(Object.keys(locales)));
-    const { code } = await terser.minify(source, { toplevel: true });
-    await fs.writeFile(`./packages/backend/built/server/web/${path.basename(file)}`, code);
-  }
+	for (const file of [
+		'./packages/backend/src/server/web/boot.js',
+		'./packages/backend/src/server/web/boot.embed.js',
+		'./packages/backend/src/server/web/bios.js',
+		'./packages/backend/src/server/web/cli.js',
+		'./packages/backend/src/server/web/error.js',
+	]) {
+		let source = await fs.readFile(file, { encoding: 'utf-8' });
+		source = source.replaceAll('LANGS', JSON.stringify(Object.keys(locales)));
+		const { code } = await terser.minify(source, { toplevel: true });
+		await fs.writeFile(`./packages/backend/built/server/web/${path.basename(file)}`, code);
+	}
 }
 
 async function buildBackendStyle() {
-  await fs.mkdir('./packages/backend/built/server/web', { recursive: true });
+	await fs.mkdir('./packages/backend/built/server/web', { recursive: true });
 
-  for (const file of [
-    './packages/backend/src/server/web/style.css',
-    './packages/backend/src/server/web/style.embed.css',
-    './packages/backend/src/server/web/bios.css',
-    './packages/backend/src/server/web/cli.css',
-    './packages/backend/src/server/web/error.css'
-  ]) {
-    const source = await fs.readFile(file, { encoding: 'utf-8' });
-    const { css } = await postcss([cssnano({ zindex: false })]).process(source, { from: undefined });
-    await fs.writeFile(`./packages/backend/built/server/web/${path.basename(file)}`, css);
-  }
+	for (const file of [
+		'./packages/backend/src/server/web/style.css',
+		'./packages/backend/src/server/web/style.embed.css',
+		'./packages/backend/src/server/web/bios.css',
+		'./packages/backend/src/server/web/cli.css',
+		'./packages/backend/src/server/web/error.css'
+	]) {
+		const source = await fs.readFile(file, { encoding: 'utf-8' });
+		const { css } = await postcss([cssnano({ zindex: false })]).process(source, { from: undefined });
+		await fs.writeFile(`./packages/backend/built/server/web/${path.basename(file)}`, css);
+	}
 }
 
 async function build() {
-  await Promise.all([
-    copyFrontendFonts(),
-    copyFrontendLocales(),
-    copyBackendViews(),
-    buildBackendScript(),
-    buildBackendStyle(),
+	await Promise.all([
+		copyFrontendFonts(),
+		copyBackendViews(),
+		buildBackendScript(),
+		buildBackendStyle(),
 		loadConfig().then(config => config?.publishTarballInsteadOfProvideRepositoryUrl && buildTarball()),
-  ]);
+	]);
 }
 
 await build();
-
-if (process.argv.includes('--watch')) {
-	const watcher = fs.watch('./locales');
-	for await (const event of watcher) {
-		const filename = event.filename?.replaceAll('\\', '/');
-		if (/^[a-z]+-[A-Z]+\.yml/.test(filename)) {
-			console.log(`update ${filename} ...`)
-			locales = buildLocales();
-			await copyFrontendLocales()
-		}
-	}
-}
diff --git a/scripts/clean-all.js b/scripts/clean-all.js
index 5a8f9eba23..839ea3ba1c 100644
--- a/scripts/clean-all.js
+++ b/scripts/clean-all.js
@@ -24,6 +24,9 @@ const fs = require('fs');
 	fs.rmSync(__dirname + '/../packages/sw/built', { recursive: true, force: true });
 	fs.rmSync(__dirname + '/../packages/sw/node_modules', { recursive: true, force: true });
 
+	fs.rmSync(__dirname + '/../packages/i18n/built', { recursive: true, force: true });
+	fs.rmSync(__dirname + '/../packages/i18n/node_modules', { recursive: true, force: true });
+
 	fs.rmSync(__dirname + '/../packages/misskey-js/built', { recursive: true, force: true });
 	fs.rmSync(__dirname + '/../packages/misskey-js/node_modules', { recursive: true, force: true });
 
diff --git a/scripts/clean.js b/scripts/clean.js
index 69a8df76af..5cce8bacab 100644
--- a/scripts/clean.js
+++ b/scripts/clean.js
@@ -11,6 +11,7 @@ const fs = require('fs');
 	fs.rmSync(__dirname + '/../packages/frontend/built', { recursive: true, force: true });
 	fs.rmSync(__dirname + '/../packages/frontend-embed/built', { recursive: true, force: true });
 	fs.rmSync(__dirname + '/../packages/icons-subsetter/built', { recursive: true, force: true });
+	fs.rmSync(__dirname + '/../packages/i18n/built', { recursive: true, force: true });
 	fs.rmSync(__dirname + '/../packages/sw/built', { recursive: true, force: true });
 	fs.rmSync(__dirname + '/../packages/misskey-js/built', { recursive: true, force: true });
 	fs.rmSync(__dirname + '/../packages/misskey-reversi/built', { recursive: true, force: true });
diff --git a/scripts/dev.mjs b/scripts/dev.mjs
index e500510b9e..b54004132a 100644
--- a/scripts/dev.mjs
+++ b/scripts/dev.mjs
@@ -16,6 +16,13 @@ await execa('pnpm', ['clean'], {
 	stderr: process.stderr,
 });
 
+// アセットのビルドで依存しているので一番最初に必要
+await execa('pnpm', ['--filter', 'i18n', 'build'], {
+	cwd: _dirname + '/../',
+	stdout: process.stdout,
+	stderr: process.stderr,
+});
+
 await Promise.all([
 	execa('pnpm', ['build-pre'], {
 		cwd: _dirname + '/../',
@@ -38,6 +45,11 @@ await Promise.all([
 		stdout: process.stdout,
 		stderr: process.stderr,
 	}),
+	execa('pnpm', ['--filter', 'misskey-js', 'build'], {
+		cwd: _dirname + '/../',
+		stdout: process.stdout,
+		stderr: process.stderr,
+	}),
 ]);
 
 execa('pnpm', ['build-pre', '--watch'], {
@@ -88,6 +100,12 @@ execa('pnpm', ['--filter', 'misskey-js', 'watch', '--no-clean'], {
 	stderr: process.stderr,
 });
 
+execa('pnpm', ['--filter', 'i18n', 'watch', '--no-clean'], {
+	cwd: _dirname + '/../',
+	stdout: process.stdout,
+	stderr: process.stderr,
+});
+
 execa('pnpm', ['--filter', 'misskey-reversi', 'watch', '--no-clean'], {
 	cwd: _dirname + '/../',
 	stdout: process.stdout,