feat(i18n): add i18n infrastructure + Simplified Chinese, localize Settings screen (refs #68) (#97)

- Add expo-localization, i18next, react-i18next (versions aligned with
  Expo 54 / RN 0.81)
- src/lib/i18n/locale-resolve.ts: pure locale-resolution helpers
  (system tag -> supported catalog, with en fallback), unit-tested via
  node --test with no RN imports
- src/lib/i18n/config.ts: i18next init wired to expo-localization
  device detection, en.json + zh-Hans.json catalogs
- Persist a locale preference (system | en | zh-Hans) in the settings
  zustand store, applied immediately via i18next.changeLanguage
- Wire I18nextProvider in app/_layout.tsx
- Localize the Settings screen (~28 strings) as the reference pattern
  for extracting user-facing strings, with a language picker row and
  reviewed Simplified Chinese translations

Other screens (session/[id], connection/*, index, chat components)
are deferred follow-up — issue #68 stays open for that work.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/lib/i18n/config.ts

@@ -0,0 +1,35 @@
+// i18next setup. Kept separate from locale-resolve.ts (the pure, testable
+// piece) because this module pulls in expo-localization + i18next, which
+// have no resolver outside Metro/RN — the same split used for buildAuth()
+// in auth.ts and the settings store's clamp/merge helpers.
+import i18next from "i18next"
+import { initReactI18next } from "react-i18next"
+import * as Localization from "expo-localization"
+import en from "./en.json"
+import zhHans from "./zh-Hans.json"
+import { resolveLocale, FALLBACK_LOCALE, type LocalePreference } from "./locale-resolve"
+
+const resources = {
+  en: { translation: en },
+  "zh-Hans": { translation: zhHans },
+}
+
+/** Device locale tags in priority order, e.g. ["zh-Hans-CN", "en-US"]. */
+export function deviceLocaleTags(): string[] {
+  return Localization.getLocales().map((locale) => locale.languageTag)
+}
+
+i18next.use(initReactI18next).init({
+  resources,
+  lng: resolveLocale("system", deviceLocaleTags()),
+  fallbackLng: FALLBACK_LOCALE,
+  interpolation: { escapeValue: false }, // React already escapes output
+})
+
+/** Apply a locale preference immediately. Persisting it is the caller's job. */
+export function setAppLocale(preference: LocalePreference): void {
+  const resolved = resolveLocale(preference, deviceLocaleTags())
+  if (i18next.language !== resolved) i18next.changeLanguage(resolved)
+}
+
+export default i18next

src/lib/i18n/en.json

@@ -0,0 +1,69 @@
+{
+  "common": {
+    "cancel": "Cancel"
+  },
+  "settings": {
+    "sections": {
+      "security": "Security",
+      "notifications": "Notifications",
+      "privacy": "Privacy",
+      "about": "About"
+    },
+    "security": {
+      "biometricOpen": {
+        "label": "Require Biometric to Open",
+        "descriptionEnabled": "Use Face ID or Touch ID to unlock the app",
+        "descriptionUnavailable": "Biometric authentication not available"
+      },
+      "biometricSend": {
+        "label": "Require Biometric to Send",
+        "description": "Authenticate before sending messages"
+      },
+      "lockNow": {
+        "label": "Lock App Now",
+        "description": "Require authentication to reopen"
+      }
+    },
+    "notifications": {
+      "disabledNotice": "Notifications are disabled at the system level. Enable them in Settings to receive alerts."
+    },
+    "privacy": {
+      "crashReporting": {
+        "label": "Crash Reporting",
+        "description": "Share anonymous crash reports via Sentry to help improve OpenCode. No code or prompts are included."
+      },
+      "privacyPolicy": {
+        "label": "Privacy Policy",
+        "description": "What data we collect and how"
+      }
+    },
+    "about": {
+      "version": "Version",
+      "github": {
+        "label": "GitHub",
+        "description": "View source code"
+      },
+      "docs": {
+        "label": "Documentation",
+        "description": "Learn how to use OpenCode"
+      }
+    },
+    "language": {
+      "label": "Language",
+      "title": "Choose Language",
+      "system": "System Default",
+      "en": "English",
+      "zhHans": "Simplified Chinese"
+    },
+    "footer": {
+      "appName": "OpenCode Mobile",
+      "tagline": "Connect to your AI coding assistant from anywhere"
+    },
+    "alerts": {
+      "privacyNotSavedTitle": "Privacy Setting Not Saved",
+      "privacyNotSavedMessage": "Crash reporting is off for this session, but your choice could not be saved. Please try again.",
+      "notificationsDisabledTitle": "Notifications Disabled",
+      "notificationsDisabledMessage": "Enable notifications for OpenCode in your device settings to receive alerts."
+    }
+  }
+}

src/lib/i18n/locale-resolve.test.ts

@@ -0,0 +1,38 @@
+import { test } from "node:test"
+import assert from "node:assert/strict"
+import { matchSupportedLocale, resolveLocale, FALLBACK_LOCALE } from "./locale-resolve.ts"
+
+test("matchSupportedLocale maps zh variants to zh-Hans", () => {
+  assert.equal(matchSupportedLocale("zh"), "zh-Hans")
+  assert.equal(matchSupportedLocale("zh-CN"), "zh-Hans")
+  assert.equal(matchSupportedLocale("zh-Hans"), "zh-Hans")
+  assert.equal(matchSupportedLocale("zh-Hans-CN"), "zh-Hans")
+  assert.equal(matchSupportedLocale("zh-Hant-TW"), "zh-Hans") // no zh-Hant catalog yet — closest match
+})
+
+test("matchSupportedLocale maps en variants to en", () => {
+  assert.equal(matchSupportedLocale("en"), "en")
+  assert.equal(matchSupportedLocale("en-US"), "en")
+  assert.equal(matchSupportedLocale("EN-GB"), "en") // case-insensitive
+})
+
+test("matchSupportedLocale returns null for unsupported languages", () => {
+  assert.equal(matchSupportedLocale("fr-FR"), null)
+  assert.equal(matchSupportedLocale("ja"), null)
+  assert.equal(matchSupportedLocale("es-ES"), null)
+})
+
+test("resolveLocale: explicit preference always wins over device tags", () => {
+  assert.equal(resolveLocale("en", ["zh-Hans-CN"]), "en")
+  assert.equal(resolveLocale("zh-Hans", ["en-US"]), "zh-Hans")
+})
+
+test("resolveLocale: system preference picks first supported device tag", () => {
+  assert.equal(resolveLocale("system", ["fr-FR", "zh-Hans-CN", "en-US"]), "zh-Hans")
+  assert.equal(resolveLocale("system", ["en-US", "zh-Hans-CN"]), "en")
+})
+
+test("resolveLocale: system preference falls back when nothing matches", () => {
+  assert.equal(resolveLocale("system", ["fr-FR", "ja-JP"]), FALLBACK_LOCALE)
+  assert.equal(resolveLocale("system", []), FALLBACK_LOCALE)
+})

src/lib/i18n/locale-resolve.ts

@@ -0,0 +1,43 @@
+// Pure locale-resolution logic, kept free of expo-localization/i18next imports
+// so it's unit-testable with plain `node --test` (same split as
+// settings-merge.ts / store-review-policy.ts).
+
+export const SUPPORTED_LOCALES = ["en", "zh-Hans"] as const
+export type SupportedLocale = (typeof SUPPORTED_LOCALES)[number]
+
+export const FALLBACK_LOCALE: SupportedLocale = "en"
+
+// User-facing preference: "system" defers to the device's locale list.
+export type LocalePreference = "system" | SupportedLocale
+
+/**
+ * Map one device locale tag (e.g. "zh-Hans-CN", "zh-CN", "en-US", "fr-FR") to
+ * one of our supported catalogs, matching by language subtag since we don't
+ * ship per-region variants. Returns null if we have no catalog for it.
+ */
+export function matchSupportedLocale(tag: string): SupportedLocale | null {
+  const lower = tag.toLowerCase()
+  if (lower.startsWith("zh")) return "zh-Hans"
+  if (lower.startsWith("en")) return "en"
+  return null
+}
+
+/**
+ * Resolve the effective app locale from a user preference plus the device's
+ * ranked list of locale tags (as returned by expo-localization's
+ * `getLocales().map(l => l.languageTag)`).
+ * - An explicit preference (anything but "system") always wins.
+ * - "system" walks the device list in priority order and uses the first tag
+ *   we have a catalog for.
+ * - If nothing matches (or the list is empty), falls back to FALLBACK_LOCALE.
+ */
+export function resolveLocale(preference: LocalePreference, deviceTags: readonly string[]): SupportedLocale {
+  if (preference !== "system") return preference
+
+  for (const tag of deviceTags) {
+    const match = matchSupportedLocale(tag)
+    if (match) return match
+  }
+
+  return FALLBACK_LOCALE
+}

src/lib/i18n/zh-Hans.json

@@ -0,0 +1,69 @@
+{
+  "common": {
+    "cancel": "取消"
+  },
+  "settings": {
+    "sections": {
+      "security": "安全",
+      "notifications": "通知",
+      "privacy": "隐私",
+      "about": "关于"
+    },
+    "security": {
+      "biometricOpen": {
+        "label": "解锁时需要生物识别",
+        "descriptionEnabled": "使用 Face ID 或 Touch ID 解锁应用",
+        "descriptionUnavailable": "生物识别功能不可用"
+      },
+      "biometricSend": {
+        "label": "发送消息时需要生物识别",
+        "description": "发送消息前需要验证身份"
+      },
+      "lockNow": {
+        "label": "立即锁定应用",
+        "description": "重新打开时需要验证身份"
+      }
+    },
+    "notifications": {
+      "disabledNotice": "通知已在系统层面关闭。请在系统设置中启用通知以接收提醒。"
+    },
+    "privacy": {
+      "crashReporting": {
+        "label": "崩溃报告",
+        "description": "通过 Sentry 分享匿名崩溃报告，帮助改进 OpenCode。报告不包含代码或提示词内容。"
+      },
+      "privacyPolicy": {
+        "label": "隐私政策",
+        "description": "我们收集哪些数据及收集方式"
+      }
+    },
+    "about": {
+      "version": "版本",
+      "github": {
+        "label": "GitHub",
+        "description": "查看源代码"
+      },
+      "docs": {
+        "label": "文档",
+        "description": "了解如何使用 OpenCode"
+      }
+    },
+    "language": {
+      "label": "语言",
+      "title": "选择语言",
+      "system": "跟随系统",
+      "en": "English",
+      "zhHans": "简体中文"
+    },
+    "footer": {
+      "appName": "OpenCode Mobile",
+      "tagline": "随时随地连接您的 AI 编程助手"
+    },
+    "alerts": {
+      "privacyNotSavedTitle": "隐私设置未保存",
+      "privacyNotSavedMessage": "本次会话已关闭崩溃报告，但您的选择未能保存。请重试。",
+      "notificationsDisabledTitle": "通知已关闭",
+      "notificationsDisabledMessage": "请在设备设置中为 OpenCode 启用通知以接收提醒。"
+    }
+  }
+}

src/stores/settings.ts

@@ -2,17 +2,21 @@ import { create } from "zustand"
 import * as SecureStore from "expo-secure-store"
 import { type Category, defaultPreferences } from "../lib/notifications"
 import { clampPageSize, mergeStoredSettings } from "../lib/settings-merge"
+import { setAppLocale } from "../lib/i18n/config"
+import type { LocalePreference } from "../lib/i18n/locale-resolve"
 
 const SETTINGS_KEY = "opencode_settings"
 
 interface Settings {
   pageSize: number
   notifications: Record<Category, boolean>
+  locale: LocalePreference
 }
 
 const DEFAULTS: Settings = {
   pageSize: 25,
   notifications: { ...defaultPreferences },
+  locale: "system",
 }
 
 interface SettingsState extends Settings {
@@ -20,10 +24,11 @@ interface SettingsState extends Settings {
   load: () => Promise<void>
   setPageSize: (size: number) => Promise<void>
   setNotification: (category: Category, enabled: boolean) => Promise<void>
+  setLocale: (locale: LocalePreference) => Promise<void>
 }
 
 function snapshot(get: () => SettingsState): Settings {
-  return { pageSize: get().pageSize, notifications: get().notifications }
+  return { pageSize: get().pageSize, notifications: get().notifications, locale: get().locale }
 }
 
 async function persist(settings: Settings) {
@@ -39,7 +44,9 @@ export const useSettings = create<SettingsState>((set, get) => ({
     if (raw) {
       const parsed = JSON.parse(raw) as Partial<Settings>
       // Merge stored settings with defaults so new fields/categories get their default
-      set({ ...mergeStoredSettings(DEFAULTS, parsed), loaded: true })
+      const merged = mergeStoredSettings(DEFAULTS, parsed)
+      set({ ...merged, loaded: true })
+      setAppLocale(merged.locale)
       return
     }
     set({ loaded: true })
@@ -56,4 +63,10 @@ export const useSettings = create<SettingsState>((set, get) => ({
     set({ notifications })
     await persist({ ...snapshot(get), notifications })
   },
+
+  setLocale: async (locale) => {
+    set({ locale })
+    setAppLocale(locale) // applies immediately
+    await persist({ ...snapshot(get), locale })
+  },
 }))