feat(i18n): add frontend internationalization setup

app/src/i18n/index.ts

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+import { createI18n } from 'vue-i18n'
+import en from './locales/en.json'
+import zhTW from './locales/zh-TW.json'
+
+export const supportedLocales = ['zh-TW', 'en'] as const
+
+export type SupportedLocale = (typeof supportedLocales)[number]
+
+function isSupportedLocale(locale: string): locale is SupportedLocale {
+  return supportedLocales.includes(locale as SupportedLocale)
+}
+
+function getInitialLocale(): SupportedLocale {
+  const savedLocale = localStorage.getItem('locale')
+
+  if (savedLocale && isSupportedLocale(savedLocale)) {
+    return savedLocale
+  }
+
+  return navigator.language.startsWith('zh') ? 'zh-TW' : 'en'
+}
+
+const initialLocale = getInitialLocale()
+
+document.documentElement.lang = initialLocale
+
+export const i18n = createI18n({
+  legacy: false,
+  locale: initialLocale,
+  fallbackLocale: 'zh-TW',
+  messages: {
+    'zh-TW': zhTW,
+    en,
+  },
+})
+
+export function setLocale(locale: SupportedLocale) {
+  i18n.global.locale.value = locale
+  localStorage.setItem('locale', locale)
+  document.documentElement.lang = locale
+}

app/src/i18n/locales/en.json

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+{}

app/src/i18n/locales/zh-TW.json

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+{}