feat(desktop): add Japanese, Korean, and Traditional Chinese locales

Add jp / kr / zh-TW translations and wire them into the i18n runtime so the
desktop app can switch among English, Simplified Chinese, Traditional Chinese,
Japanese, and Korean.

- new locale files jp.ts / kr.ts / zh-TW.ts with full TranslationKey coverage
- register the locales in the i18n index and extend the Locale union
- accept the new codes in settingsStore.getStoredLocale (default stays zh)
- add language-switcher entries (简体中文 / 繁體中文 / 日本語 / 한국어)
- localize chat message timestamps: Han 年月日 for zh/zh-TW/jp, Intl ko-KR for kr
- extend the DATE_LOCALES map in ActivitySettings (ja-JP / ko-KR / zh-TW)
- tests: formatMessageTimestamp locale branches + i18n locale resolution

Korean terminology follows the Microsoft Korean localization style guide
(새로 고침 / 사용·사용 안 함 / 이름 바꾸기, 합니다-style sentences).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

desktop/src/i18n/index.test.tsx

@@ -1,7 +1,7 @@
 import { act, renderHook } from '@testing-library/react'
 import { afterEach, describe, expect, it } from 'vitest'
 import { useSettingsStore } from '../stores/settingsStore'
-import { useTranslation } from '.'
+import { translate, useTranslation } from '.'
 
 describe('useTranslation', () => {
   afterEach(() => {
@@ -26,4 +26,17 @@ describe('useTranslation', () => {
     })
     expect(result.current).not.toBe(initial)
   })
+
+  it('resolves every registered locale to its own translation', () => {
+    expect(translate('en', 'common.save')).toBe('Save')
+    expect(translate('zh', 'common.save')).toBe('保存')
+    expect(translate('zh-TW', 'common.save')).toBe('儲存')
+    expect(translate('jp', 'common.save')).toBe('保存')
+    expect(translate('kr', 'common.save')).toBe('저장')
+  })
+
+  it('interpolates params across the new locales', () => {
+    expect(translate('jp', 'session.timeMinutes', { n: 5 })).toBe('5 分前')
+    expect(translate('kr', 'session.timeMinutes', { n: 5 })).toBe('5분 전')
+  })
 })

desktop/src/i18n/index.ts

@@ -2,10 +2,19 @@ import { useCallback } from 'react'
 import { useSettingsStore } from '../stores/settingsStore'
 import { en, type TranslationKey } from './locales/en'
 import { zh } from './locales/zh'
+import { zh as zhTW } from './locales/zh-TW'
+import { jp } from './locales/jp'
+import { kr } from './locales/kr'
 
-export type Locale = 'en' | 'zh'
+export type Locale = 'en' | 'zh' | 'zh-TW' | 'jp' | 'kr'
 
-const translations: Record<Locale, Record<string, string>> = { en, zh }
+const translations: Record<Locale, Record<string, string>> = {
+  en,
+  zh,
+  'zh-TW': zhTW,
+  jp,
+  kr,
+}
 
 /**
  * Translate a key with optional interpolation params.

desktop/src/lib/formatMessageTimestamp.test.ts

@@ -26,4 +26,34 @@ describe('formatMessageTimestamp', () => {
 
     expect(formatMessageTimestamp(value, t('zh'), 'zh', now)).toBe('4月20日 09:30')
   })
+
+  it('formats Japanese history with Han year/month/day characters', () => {
+    const weekday = new Date(2026, 4, 24, 15, 50).getTime()
+    expect(formatMessageTimestamp(weekday, t('jp'), 'jp', now)).toContain('15:50')
+
+    const monthDay = new Date(2026, 3, 20, 9, 30).getTime()
+    expect(formatMessageTimestamp(monthDay, t('jp'), 'jp', now)).toBe('4月20日 09:30')
+
+    const yearMonthDay = new Date(2025, 11, 15, 9, 30).getTime()
+    expect(formatMessageTimestamp(yearMonthDay, t('jp'), 'jp', now)).toBe('2025年12月15日 09:30')
+
+    expect(formatMessageTimestamp(now - 5 * 60_000, t('jp'), 'jp', now)).toBe('5 分前')
+  })
+
+  it('formats Traditional Chinese history with Han year/month/day characters', () => {
+    const monthDay = new Date(2026, 3, 20, 9, 30).getTime()
+    expect(formatMessageTimestamp(monthDay, t('zh-TW'), 'zh-TW', now)).toBe('4月20日 09:30')
+
+    const yearMonthDay = new Date(2025, 11, 15, 9, 30).getTime()
+    expect(formatMessageTimestamp(yearMonthDay, t('zh-TW'), 'zh-TW', now)).toBe('2025年12月15日 09:30')
+  })
+
+  it('formats Korean history via Intl (ko-KR) without Han characters', () => {
+    const monthDay = new Date(2026, 3, 20, 9, 30).getTime()
+    const out = formatMessageTimestamp(monthDay, t('kr'), 'kr', now)
+    expect(out).toContain('09:30')
+    expect(out).not.toContain('月') // Korean uses 월, never the Han 月
+
+    expect(formatMessageTimestamp(now - 5 * 60_000, t('kr'), 'kr', now)).toBe('5분 전')
+  })
 })

desktop/src/lib/formatMessageTimestamp.ts

@@ -52,19 +52,32 @@ function coerceDate(value: number | string | Date): Date | null {
 }
 
 function localeToIntl(locale: Locale): string {
-  return locale === 'zh' ? 'zh-CN' : 'en-US'
+  switch (locale) {
+    case 'zh': return 'zh-CN'
+    case 'zh-TW': return 'zh-TW'
+    case 'jp': return 'ja-JP'
+    case 'kr': return 'ko-KR'
+    default: return 'en-US'
+  }
+}
+
+// Locales whose dates use the 年/月/日 Han characters: Chinese (Simplified/Traditional)
+// and Japanese share identical glyphs. Korean uses 년/월/일, so it goes through Intl instead.
+function usesHanYmd(locale: Locale): boolean {
+  return locale === 'zh' || locale === 'zh-TW' || locale === 'jp'
 }
 
 function formatWeekdayTime(date: Date, locale: Locale): string {
   const intlLocale = localeToIntl(locale)
-  const weekday = new Intl.DateTimeFormat(intlLocale, { weekday: locale === 'zh' ? 'long' : 'short' }).format(date)
+  const han = usesHanYmd(locale)
+  const weekday = new Intl.DateTimeFormat(intlLocale, { weekday: han ? 'long' : 'short' }).format(date)
   const time = formatClockTime(date, intlLocale)
-  return locale === 'zh' ? `${weekday}${time}` : `${weekday} ${time}`
+  return han ? `${weekday}${time}` : `${weekday} ${time}`
 }
 
 function formatMonthDayTime(date: Date, locale: Locale): string {
   const intlLocale = localeToIntl(locale)
-  if (locale === 'zh') {
+  if (usesHanYmd(locale)) {
     return `${date.getMonth() + 1}月${date.getDate()}日 ${formatClockTime(date, intlLocale)}`
   }
   const day = new Intl.DateTimeFormat(intlLocale, {
@@ -76,7 +89,7 @@ function formatMonthDayTime(date: Date, locale: Locale): string {
 
 function formatYearMonthDayTime(date: Date, locale: Locale): string {
   const intlLocale = localeToIntl(locale)
-  if (locale === 'zh') {
+  if (usesHanYmd(locale)) {
     return `${date.getFullYear()}年${date.getMonth() + 1}月${date.getDate()}日 ${formatClockTime(date, intlLocale)}`
   }
   const day = new Intl.DateTimeFormat(intlLocale, {

desktop/src/pages/ActivitySettings.tsx

@@ -50,6 +50,9 @@ const HEAT_COLORS = [
 const DATE_LOCALES: Record<Locale, string> = {
   en: 'en-US',
   zh: 'zh-CN',
+  'zh-TW': 'zh-TW',
+  jp: 'ja-JP',
+  kr: 'ko-KR',
 }
 const DEFAULT_PROFILE: DesktopProfilePreferences = {
   displayName: 'cc-haha',

desktop/src/pages/Settings.tsx

@@ -1517,9 +1517,13 @@ function GeneralSettings() {
 
   const LANGUAGES: Array<{ value: Locale; label: string }> = [
     { value: 'en', label: 'English' },
-    { value: 'zh', label: '中文' },
+    { value: 'zh', label: '简体中文' },
+    { value: 'zh-TW', label: '繁體中文' },
+    { value: 'jp', label: '日本語' },
+    { value: 'kr', label: '한국어' },
   ]
 
+
   const RESPONSE_LANGUAGES: Array<{ value: string; label: string }> = [
     { value: '', label: t('settings.general.responseLangDefault') },
     { value: 'english', label: 'English' },

desktop/src/stores/settingsStore.ts

@@ -41,10 +41,12 @@ export const UI_ZOOM_STEP = APP_ZOOM_CONTROL_STEP
 export const UI_ZOOM_DEFAULT = DEFAULT_APP_ZOOM
 let desktopNotificationsSaveQueue: Promise<void> = Promise.resolve()
 
+const VALID_LOCALES: readonly Locale[] = ['en', 'zh', 'zh-TW', 'jp', 'kr']
+
 function getStoredLocale(): Locale {
   try {
     const stored = localStorage.getItem(LOCALE_STORAGE_KEY)
-    if (stored === 'en' || stored === 'zh') return stored
+    if (stored && (VALID_LOCALES as readonly string[]).includes(stored)) return stored as Locale
   } catch { /* localStorage unavailable */ }
   return 'zh'
 }