feat(frontend): 实现中英文切换与拼写检查

frontend/src/i18n.ts

@@ -0,0 +1,28 @@
+import { ref, watch } from 'vue'
+
+export type AppLocale = 'zh-CN' | 'en'
+
+function storedLocale(): AppLocale {
+  if (typeof localStorage === 'undefined') return 'zh-CN'
+  try {
+    const saved = JSON.parse(localStorage.getItem('app-settings') ?? '{}') as { language?: unknown }
+    return saved.language === 'en' ? 'en' : 'zh-CN'
+  } catch {
+    return 'zh-CN'
+  }
+}
+
+export const appLocale = ref<AppLocale>(storedLocale())
+
+watch(appLocale, (value) => {
+  if (typeof document !== 'undefined') document.documentElement.lang = value
+}, { immediate: true })
+
+/** Keep the Chinese source beside its English translation while the UI is migrated. */
+export function t(zh: string, en: string): string {
+  return appLocale.value === 'en' ? en : zh
+}
+
+export function localeTag(): string {
+  return appLocale.value === 'en' ? 'en' : 'zh-CN'
+}