feat: remove Japanese language support (English-only)

- Remove Japanese translations from lib/i18n/translations.ts
- Simplify I18nProvider to English-only in lib/i18n/context.tsx
- Remove locale state from settingsSlice
- Remove language selector UI from Settings page
- Update middleware tests for English-only locale
- Net reduction: ~253 lines of code

Author: ryota-murakami

app/settings/SettingsClient.tsx

@@ -6,7 +6,7 @@
 
 'use client'
 
-import { Check, Sun, Moon, Monitor, Globe } from 'lucide-react'
+import { Check, Sun, Moon, Monitor } from 'lucide-react'
 import Link from 'next/link'
 import { useRouter } from 'next/navigation'
 import { useState, memo, useMemo, useCallback } from 'react'
@@ -88,13 +88,6 @@ const THEME_BUTTON_SELECTED =
 const THEME_BUTTON_UNSELECTED =
   'border-border hover:border-primary/50 hover:bg-muted/50'
 
-/** Base styles for language button */
-const LANGUAGE_BUTTON_BASE =
-  'flex items-center gap-2 rounded-lg border-2 px-4 py-3 transition-all'
-const LANGUAGE_BUTTON_SELECTED =
-  'border-primary bg-primary/5 ring-2 ring-primary ring-offset-2'
-const LANGUAGE_BUTTON_UNSELECTED = 'border-border hover:border-primary/50'
-
 const THEME_INFO: Record<
   ThemeType,
   { name: string; color: string; description: string }
@@ -169,7 +162,7 @@ const ThemeButton = memo(function ThemeButton({
 export const SettingsClient = memo(function SettingsClient() {
   const router = useRouter()
   const { theme, setTheme, mounted } = useTheme()
-  const { language, setLanguage, t } = useI18n()
+  const { t } = useI18n()
 
   /**
    * Handles the save settings action.
@@ -182,18 +175,6 @@ export const SettingsClient = memo(function SettingsClient() {
     router.push('/boards')
   }, [router])
 
-  const englishButtonClassName = useMemo(
-    () =>
-      `${LANGUAGE_BUTTON_BASE} ${language === 'en' ? LANGUAGE_BUTTON_SELECTED : LANGUAGE_BUTTON_UNSELECTED}`,
-    [language],
-  )
-
-  const japaneseButtonClassName = useMemo(
-    () =>
-      `${LANGUAGE_BUTTON_BASE} ${language === 'ja' ? LANGUAGE_BUTTON_SELECTED : LANGUAGE_BUTTON_UNSELECTED}`,
-    [language],
-  )
-
   if (!mounted) {
     return (
       <div className="container mx-auto max-w-4xl px-4 py-8">
@@ -349,51 +330,6 @@ export const SettingsClient = memo(function SettingsClient() {
           </CardContent>
         </Card>
 
-        {/* Language Settings */}
-        <Card>
-          <CardHeader>
-            <CardTitle className="flex items-center gap-2">
-              <Globe className="h-5 w-5" />
-              Language / 言語
-            </CardTitle>
-            <CardDescription>Choose your preferred language</CardDescription>
-          </CardHeader>
-          <CardContent>
-            <div className="flex gap-3">
-              <button
-                type="button"
-                onClick={() => setLanguage('en')}
-                className={englishButtonClassName}
-              >
-                <span className="text-2xl">🇺🇸</span>
-                <div className="text-left">
-                  <span className="block font-medium">English</span>
-                  <span className="text-xs text-muted-foreground">English</span>
-                </div>
-                {language === 'en' && (
-                  <Check className="ml-2 h-4 w-4 text-primary" />
-                )}
-              </button>
-              <button
-                type="button"
-                onClick={() => setLanguage('ja')}
-                className={japaneseButtonClassName}
-              >
-                <span className="text-2xl">🇯🇵</span>
-                <div className="text-left">
-                  <span className="block font-medium">日本語</span>
-                  <span className="text-xs text-muted-foreground">
-                    Japanese
-                  </span>
-                </div>
-                {language === 'ja' && (
-                  <Check className="ml-2 h-4 w-4 text-primary" />
-                )}
-              </button>
-            </div>
-          </CardContent>
-        </Card>
-
         {/* Actions */}
         <div className="flex justify-between pt-4">
           <Link href="/boards">

lib/i18n/context.tsx

@@ -1,102 +1,69 @@
 /**
  * Internationalization Context
  *
- * Provides language switching and translation access throughout the app
+ * Provides translation access throughout the app (English-only)
  */
 
 'use client'
 
-import React, {
-  createContext,
-  useContext,
-  useState,
-  useCallback,
-  useMemo,
-  useSyncExternalStore,
-  memo,
-} from 'react'
+import React, { createContext, useContext, useMemo, memo } from 'react'
 
-import type { Language } from './translations'
 import { translations } from './translations'
 
-const LANGUAGE_STORAGE_KEY = 'gitbox-language'
-
 interface I18nContextType {
-  language: Language
-  setLanguage: (lang: Language) => void
-  t: (typeof translations)[Language]
+  t: typeof translations.en
 }
 
 const I18nContext = createContext<I18nContextType | undefined>(undefined)
 
-// Helper to detect if we're in a browser environment
-const getSnapshot = () => true
-const getServerSnapshot = () => false
-const subscribe = () => () => {}
-
+/**
+ * Provider component that supplies English translations to the app.
+ * @param children - React children to wrap with i18n context
+ * @returns Provider component with English translations
+ */
 export const I18nProvider = memo(function I18nProvider({
   children,
 }: {
   children: React.ReactNode
 }): React.ReactNode {
-  const isClient = useSyncExternalStore(
-    subscribe,
-    getSnapshot,
-    getServerSnapshot,
-  )
-
-  // Initialize language from localStorage (client-side only)
-  const [language, setLanguageState] = useState<Language>(() => {
-    if (typeof window === 'undefined') return 'en'
-    const stored = localStorage.getItem(LANGUAGE_STORAGE_KEY) as Language | null
-    if (stored && (stored === 'en' || stored === 'ja')) {
-      return stored
-    }
-    const browserLang = navigator.language.split('-')[0]
-    return browserLang === 'ja' ? 'ja' : 'en'
-  })
-
-  const setLanguage = useCallback((lang: Language) => {
-    setLanguageState(lang)
-    localStorage.setItem(LANGUAGE_STORAGE_KEY, lang)
-    document.documentElement.lang = lang
-  }, [])
-
-  // Memoize context value for stable reference
   const contextValue = useMemo(
     () => ({
-      language,
-      setLanguage,
-      t: translations[language],
+      t: translations.en,
     }),
-    [language, setLanguage],
+    [],
   )
 
-  // Show children with default English until client-side hydration
-  if (!isClient) {
-    return children
-  }
-
   return (
     <I18nContext.Provider value={contextValue}>{children}</I18nContext.Provider>
   )
 })
 
+/**
+ * Hook to access i18n context with translations.
+ * @returns Object containing English translations
+ * @example
+ * const { t } = useI18n()
+ * return <h1>{t.common.loading}</h1>
+ */
 export function useI18n() {
   const context = useContext(I18nContext)
   if (!context) {
     // Return default values for SSR/pre-render
     return {
-      language: 'en' as Language,
-      setLanguage: () => {},
       t: translations.en,
     }
   }
   return context
 }
 
-// Utility hook for just getting translations (simpler API)
+/**
+ * Utility hook for getting translations (simpler API).
+ * @returns Object containing translations
+ * @example
+ * const { t } = useTranslation()
+ * return <span>{t.settings.title}</span>
+ */
 export function useTranslation() {
-  const { t, language } = useI18n()
-  return { t, language }
+  const { t } = useI18n()
+  return { t }
 }

lib/i18n/index.ts

@@ -2,9 +2,5 @@
  * i18n Module Exports
  */
 
-export {
-  translations,
-  type Language,
-  type TranslationKeys,
-} from './translations'
+export { translations, type TranslationKeys } from './translations'
 export { I18nProvider, useI18n, useTranslation } from './context'

lib/i18n/translations.ts

@@ -1,7 +1,7 @@
 /**
  * Internationalization Translations
  *
- * PRD: 対応言語 - 英語、日本語
+ * English-only translation system
  */
 
 export const translations = {
@@ -145,146 +145,6 @@ export const translations = {
       followSystem: 'Follow system preference',
     },
   },
-  ja: {
-    common: {
-      loading: '読み込み中...',
-      error: 'エラー',
-      save: '保存',
-      cancel: 'キャンセル',
-      delete: '削除',
-      edit: '編集',
-      create: '作成',
-      back: '戻る',
-      signIn: 'ログイン',
-      signOut: 'ログアウト',
-      search: '検索',
-      settings: '設定',
-      help: 'ヘルプ',
-    },
-    nav: {
-      boards: 'ボード',
-      allBoards: 'すべてのボード',
-      favorites: 'お気に入り',
-      maintenance: 'メンテナンスモード',
-      settings: '設定',
-      shortcuts: 'ショートカット',
-    },
-    landing: {
-      title: 'GitHubリポジトリを今までにない方法で管理',
-      subtitle: 'GitHubリポジトリを整理するための美しいカンバンボード',
-      getStarted: '始める',
-      signIn: 'GitHubでログイン',
-    },
-    boards: {
-      title: 'マイボード',
-      description: 'GitHubリポジトリをカンバン形式で管理',
-      createBoard: 'ボードを作成',
-      noBoards: 'ボードがありません',
-      noBoardsDescription: '最初のボードを作成して始めましょう',
-    },
-    board: {
-      addColumn: '列を追加',
-      settings: 'ボード設定',
-      wipLimit: 'WIP制限',
-      wipExceeded: 'WIP制限を超えています',
-    },
-    card: {
-      openGitHub: 'GitHubで開く',
-      openProduction: '本番URLを開く',
-      openTracking: 'トラッキングダッシュボードを開く',
-      openSupabase: 'Supabaseダッシュボードを開く',
-      editProjectInfo: 'プロジェクト情報を編集…',
-      moveToMaintenance: 'メンテナンスに移動',
-      restoreToBoard: 'ボードに復元',
-    },
-    maintenance: {
-      title: 'メンテナンスモード',
-      description: 'アーカイブ済み・保守中のプロジェクト',
-      items: '件',
-      noProjects: 'メンテナンスプロジェクトがありません',
-      noProjectsDescription:
-        'ボードからプロジェクトをここに移動してアーカイブできます',
-      searchRepos: 'リポジトリを検索...',
-    },
-    settings: {
-      title: '設定',
-      description: 'GitBoxをカスタマイズ',
-      theme: 'テーマ',
-      themeDescription: 'お好みのテーマを選択してください',
-      system: 'システム',
-      lightThemes: 'ライトテーマ',
-      darkThemes: 'ダークテーマ',
-      display: '表示',
-      displayDescription: 'コンテンツの表示方法を設定',
-      compactMode: 'コンパクトモード',
-      compactModeDescription: '間隔とカードサイズを縮小',
-      showMetadata: 'カードメタデータを表示',
-      showMetadataDescription: 'スター数、言語、最終更新日を表示',
-      wipWarnings: 'WIP制限警告',
-      wipWarningsDescription: 'WIP制限超過時に警告を表示',
-      typography: 'タイポグラフィ',
-      typographyDescription: '読みやすさのためにテキストサイズを調整',
-      fontSize: '基本フォントサイズ',
-      backToBoards: 'ボードに戻る',
-      saveSettings: '設定を保存',
-    },
-    commandPalette: {
-      searchCommands: 'コマンドを検索...',
-      navigation: 'ナビゲーション',
-      actions: 'アクション',
-      goToBoards: 'ボードへ移動',
-      goToMaintenance: 'メンテナンスへ移動',
-      goToSettings: '設定へ移動',
-      createNewBoard: '新規ボード作成',
-      changeTheme: 'テーマを変更',
-      keyboardShortcuts: 'キーボードショートカット',
-      helpDocs: 'ヘルプとドキュメント',
-    },
-    createBoard: {
-      title: '新規ボード作成',
-      description: 'ボード名を入力してテーマを選択してください',
-      boardName: 'ボード名',
-      boardNamePlaceholder: '例: AIプロジェクト, サイドプロジェクト',
-      theme: 'テーマ',
-      light: 'ライト',
-      dark: 'ダーク',
-      creating: '作成中...',
-    },
-    projectInfo: {
-      title: 'プロジェクト情報',
-      note: 'ノート',
-      links: 'リンク',
-      credentials: '認証情報',
-      addLink: 'リンクを追加',
-      addCredential: '認証情報を追加',
-    },
-    statusList: {
-      addColumn: '新しい列を追加',
-      editColumn: '列を編集',
-      name: '名前',
-      color: '色',
-      wipLimit: 'WIP制限 (オプション)',
-      wipLimitDescription:
-        '作業中アイテムの制限。制限なしの場合は空にしてください。',
-      saveChanges: '変更を保存',
-    },
-    themes: {
-      sunrise: 'サンライズ',
-      sandstone: 'サンドストーン',
-      mint: 'ミント',
-      sky: 'スカイ',
-      lavender: 'ラベンダー',
-      rose: 'ローズ',
-      midnight: 'ミッドナイト',
-      graphite: 'グラファイト',
-      forest: 'フォレスト',
-      ocean: 'オーシャン',
-      plum: 'プラム',
-      rust: 'ラスト',
-      followSystem: 'システム設定に従う',
-    },
-  },
 } as const
 
-export type Language = keyof typeof translations
 export type TranslationKeys = typeof translations.en