diff --git a/app.json b/app.json
index bd7e85d..7249f49 100644
--- a/app.json
+++ b/app.json
@@ -79,7 +79,8 @@
           "microphonePermission": "OpenCode uses the microphone to capture your voice when using speech-to-text input.",
           "speechRecognitionPermission": "OpenCode uses speech recognition so you can dictate prompts to your AI coding agent hands-free."
         }
-      ]
+      ],
+      "expo-localization"
     ],
     "experiments": {
       "typedRoutes": true,
diff --git a/app/(tabs)/settings.tsx b/app/(tabs)/settings.tsx
index d6395e7..d936f37 100644
--- a/app/(tabs)/settings.tsx
+++ b/app/(tabs)/settings.tsx
@@ -11,6 +11,7 @@ import {
   Alert,
 } from "react-native"
 import { Ionicons } from "@expo/vector-icons"
+import { useTranslation } from "react-i18next"
 import { useAuth } from "../../src/stores/auth"
 import { useSettings } from "../../src/stores/settings"
 import {
@@ -22,6 +23,7 @@ import {
 import type { Category } from "../../src/lib/notifications"
 import { hasTelemetryConsent, setTelemetryConsent } from "../../src/lib/telemetry"
 import { PRIVACY_POLICY_URL } from "../../src/lib/links"
+import type { LocalePreference } from "../../src/lib/i18n/locale-resolve"
 
 function SettingRow({
   icon,
@@ -70,9 +72,10 @@ function SettingSection({ title, children, isDark }: { title: string; children:
 export default function SettingsScreen() {
   const colorScheme = useColorScheme()
   const isDark = colorScheme === "dark"
+  const { t } = useTranslation()
 
   const { settings, hasBiometrics, updateSettings, lock } = useAuth()
-  const { notifications, setNotification } = useSettings()
+  const { notifications, setNotification, locale, setLocale } = useSettings()
   const [osGranted, setOsGranted] = useState<boolean | null>(null)
   const [telemetryUpdating, setTelemetryUpdating] = useState(false)
 
@@ -80,21 +83,21 @@ export default function SettingsScreen() {
   // We initialise local state from in-memory value; updates call setTelemetryConsent().
   const [crashReporting, setCrashReporting] = useState<boolean>(hasTelemetryConsent() ?? false)
 
-  const handleCrashReportingToggle = useCallback(async (value: boolean) => {
-    setTelemetryUpdating(true)
-    try {
-      await setTelemetryConsent(value)
-      setCrashReporting(value)
-    } catch {
-      setCrashReporting(hasTelemetryConsent() ?? false)
-      Alert.alert(
-        "Privacy Setting Not Saved",
-        "Crash reporting is off for this session, but your choice could not be saved. Please try again.",
-      )
-    } finally {
-      setTelemetryUpdating(false)
-    }
-  }, [])
+  const handleCrashReportingToggle = useCallback(
+    async (value: boolean) => {
+      setTelemetryUpdating(true)
+      try {
+        await setTelemetryConsent(value)
+        setCrashReporting(value)
+      } catch {
+        setCrashReporting(hasTelemetryConsent() ?? false)
+        Alert.alert(t("settings.alerts.privacyNotSavedTitle"), t("settings.alerts.privacyNotSavedMessage"))
+      } finally {
+        setTelemetryUpdating(false)
+      }
+    },
+    [t],
+  )
 
   // Check OS permission state on first toggle attempt
   const handleToggle = useCallback(
@@ -103,16 +106,13 @@ export default function SettingsScreen() {
         const ok = await setupNotifications()
         setOsGranted(ok)
         if (!ok) {
-          Alert.alert(
-            "Notifications Disabled",
-            "Enable notifications for OpenCode in your device settings to receive alerts.",
-          )
+          Alert.alert(t("settings.alerts.notificationsDisabledTitle"), t("settings.alerts.notificationsDisabledMessage"))
           return
         }
       }
       setNotification(category, enabled)
     },
-    [setNotification],
+    [setNotification, t],
   )
 
   // Lazy-check OS permission for status display
@@ -122,14 +122,31 @@ export default function SettingsScreen() {
       .catch(() => setOsGranted(false))
   }
 
+  const localeLabels: Record<LocalePreference, string> = {
+    system: t("settings.language.system"),
+    en: t("settings.language.en"),
+    "zh-Hans": t("settings.language.zhHans"),
+  }
+
+  const handleLanguagePress = useCallback(() => {
+    Alert.alert(t("settings.language.title"), undefined, [
+      { text: localeLabels.system, onPress: () => setLocale("system") },
+      { text: localeLabels.en, onPress: () => setLocale("en") },
+      { text: localeLabels["zh-Hans"], onPress: () => setLocale("zh-Hans") },
+      { text: t("common.cancel"), style: "cancel" },
+    ])
+  }, [t, setLocale, localeLabels])
+
   return (
     <ScrollView style={[styles.container, isDark && styles.containerDark]} contentContainerStyle={styles.content}>
-      <SettingSection title="Security" isDark={isDark}>
+      <SettingSection title={t("settings.sections.security")} isDark={isDark}>
         <SettingRow
           icon="finger-print"
-          label="Require Biometric to Open"
+          label={t("settings.security.biometricOpen.label")}
           description={
-            hasBiometrics ? "Use Face ID or Touch ID to unlock the app" : "Biometric authentication not available"
+            hasBiometrics
+              ? t("settings.security.biometricOpen.descriptionEnabled")
+              : t("settings.security.biometricOpen.descriptionUnavailable")
           }
           isDark={isDark}
           right={
@@ -143,8 +160,8 @@ export default function SettingsScreen() {
         />
         <SettingRow
           icon="lock-closed"
-          label="Require Biometric to Send"
-          description="Authenticate before sending messages"
+          label={t("settings.security.biometricSend.label")}
+          description={t("settings.security.biometricSend.description")}
           isDark={isDark}
           right={
             <Switch
@@ -158,8 +175,8 @@ export default function SettingsScreen() {
         {settings.requireBiometric && (
           <SettingRow
             icon="exit"
-            label="Lock App Now"
-            description="Require authentication to reopen"
+            label={t("settings.security.lockNow.label")}
+            description={t("settings.security.lockNow.description")}
             isDark={isDark}
             onPress={lock}
             right={<Ionicons name="chevron-forward" size={20} color={isDark ? "#666666" : "#999999"} />}
@@ -167,7 +184,7 @@ export default function SettingsScreen() {
         )}
       </SettingSection>
 
-      <SettingSection title="Notifications" isDark={isDark}>
+      <SettingSection title={t("settings.sections.notifications")} isDark={isDark}>
         {categories.map((category) => {
           const meta = categoryMeta[category]
           return (
@@ -190,17 +207,17 @@ export default function SettingsScreen() {
         {osGranted === false && (
           <View style={[styles.settingRow, isDark && styles.settingRowDark]}>
             <Text style={[styles.settingDescription, { color: "#ef4444", paddingLeft: 48 }]}>
-              Notifications are disabled at the system level. Enable them in Settings to receive alerts.
+              {t("settings.notifications.disabledNotice")}
             </Text>
           </View>
         )}
       </SettingSection>
 
-      <SettingSection title="Privacy" isDark={isDark}>
+      <SettingSection title={t("settings.sections.privacy")} isDark={isDark}>
         <SettingRow
           icon="shield-checkmark"
-          label="Crash Reporting"
-          description="Share anonymous crash reports via Sentry to help improve OpenCode. No code or prompts are included."
+          label={t("settings.privacy.crashReporting.label")}
+          description={t("settings.privacy.crashReporting.description")}
           isDark={isDark}
           right={
             <Switch
@@ -213,28 +230,36 @@ export default function SettingsScreen() {
         />
         <SettingRow
           icon="document-text"
-          label="Privacy Policy"
-          description="What data we collect and how"
+          label={t("settings.privacy.privacyPolicy.label")}
+          description={t("settings.privacy.privacyPolicy.description")}
           isDark={isDark}
           onPress={() => Linking.openURL(PRIVACY_POLICY_URL)}
           right={<Ionicons name="open-outline" size={20} color={isDark ? "#666666" : "#999999"} />}
         />
       </SettingSection>
 
-      <SettingSection title="About" isDark={isDark}>
-        <SettingRow icon="information-circle" label="Version" description="1.0.0" isDark={isDark} />
+      <SettingSection title={t("settings.sections.about")} isDark={isDark}>
+        <SettingRow
+          icon="language"
+          label={t("settings.language.label")}
+          description={localeLabels[locale]}
+          isDark={isDark}
+          onPress={handleLanguagePress}
+          right={<Ionicons name="chevron-forward" size={20} color={isDark ? "#666666" : "#999999"} />}
+        />
+        <SettingRow icon="information-circle" label={t("settings.about.version")} description="1.0.0" isDark={isDark} />
         <SettingRow
           icon="logo-github"
-          label="GitHub"
-          description="View source code"
+          label={t("settings.about.github.label")}
+          description={t("settings.about.github.description")}
           isDark={isDark}
           onPress={() => Linking.openURL("https://github.com/anomalyco/opencode")}
           right={<Ionicons name="open-outline" size={20} color={isDark ? "#666666" : "#999999"} />}
         />
         <SettingRow
           icon="document-text"
-          label="Documentation"
-          description="Learn how to use OpenCode"
+          label={t("settings.about.docs.label")}
+          description={t("settings.about.docs.description")}
           isDark={isDark}
           onPress={() => Linking.openURL("https://opencode.ai/docs")}
           right={<Ionicons name="open-outline" size={20} color={isDark ? "#666666" : "#999999"} />}
@@ -242,10 +267,8 @@ export default function SettingsScreen() {
       </SettingSection>
 
       <View style={styles.footer}>
-        <Text style={[styles.footerText, isDark && styles.metaDark]}>OpenCode Mobile</Text>
-        <Text style={[styles.footerText, isDark && styles.metaDark]}>
-          Connect to your AI coding assistant from anywhere
-        </Text>
+        <Text style={[styles.footerText, isDark && styles.metaDark]}>{t("settings.footer.appName")}</Text>
+        <Text style={[styles.footerText, isDark && styles.metaDark]}>{t("settings.footer.tagline")}</Text>
       </View>
     </ScrollView>
   )
diff --git a/app/_layout.tsx b/app/_layout.tsx
index ef6e758..5cb9419 100644
--- a/app/_layout.tsx
+++ b/app/_layout.tsx
@@ -5,6 +5,8 @@ import { useColorScheme, View, ActivityIndicator } from "react-native"
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query"
 import { GestureHandlerRootView } from "react-native-gesture-handler"
 import { BottomSheetModalProvider } from "@gorhom/bottom-sheet"
+import { I18nextProvider } from "react-i18next"
+import i18n from "../src/lib/i18n/config"
 import { useAuth } from "../src/stores/auth"
 import { useConnections } from "../src/stores/connections"
 import { useEvents } from "../src/stores/events"
@@ -107,6 +109,7 @@ function RootLayout() {
 
   return (
     <ErrorBoundary>
+      <I18nextProvider i18n={i18n}>
       <GestureHandlerRootView style={{ flex: 1 }}>
         <BottomSheetModalProvider>
           <QueryClientProvider client={queryClient}>
@@ -162,6 +165,7 @@ function RootLayout() {
           setConsentState("decided")
         }}
       />
+      </I18nextProvider>
     </ErrorBoundary>
   )
 }
diff --git a/package-lock.json b/package-lock.json
index c27d331..f1d88ab 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -22,14 +22,17 @@
         "expo-image-picker": "17.0.10",
         "expo-linking": "~8.0.11",
         "expo-local-authentication": "~17.0.8",
+        "expo-localization": "~17.0.9",
         "expo-notifications": "~0.32.16",
         "expo-router": "~6.0.22",
         "expo-secure-store": "~15.0.8",
         "expo-speech-recognition": "3.0.1",
         "expo-status-bar": "~3.0.9",
         "expo-store-review": "~9.0.9",
+        "i18next": "^26.3.6",
         "posthog-react-native": "^4.57.0",
         "react": "19.1.0",
+        "react-i18next": "^17.0.10",
         "react-native": "0.81.5",
         "react-native-gesture-handler": "~2.28.0",
         "react-native-marked": "8.0.0",
@@ -4921,6 +4924,19 @@
         "expo": "*"
       }
     },
+    "node_modules/expo-localization": {
+      "version": "17.0.9",
+      "resolved": "https://registry.npmjs.org/expo-localization/-/expo-localization-17.0.9.tgz",
+      "integrity": "sha512-k5eYHr7iLMob3M8cHH6bgDrDRmqnZG8xKGLhpx8ST+LsFXmSgYpJ/t0VwWm0gz64C/K669XhObdG3D6QwCGqhw==",
+      "license": "MIT",
+      "dependencies": {
+        "rtl-detect": "^1.0.2"
+      },
+      "peerDependencies": {
+        "expo": "*",
+        "react": "*"
+      }
+    },
     "node_modules/expo-modules-autolinking": {
       "version": "3.0.25",
       "resolved": "https://registry.npmjs.org/expo-modules-autolinking/-/expo-modules-autolinking-3.0.25.tgz",
@@ -6043,6 +6059,15 @@
       ],
       "license": "MIT"
     },
+    "node_modules/html-parse-stringify": {
+      "version": "3.0.1",
+      "resolved": "https://registry.npmjs.org/html-parse-stringify/-/html-parse-stringify-3.0.1.tgz",
+      "integrity": "sha512-KknJ50kTInJ7qIScF3jeaFRpMpE8/lfiTdzf/twXyPBLAGrLRTmkz3AdTnKeh40X8k9L2fdYwEp/42WGXIRGcg==",
+      "license": "MIT",
+      "dependencies": {
+        "void-elements": "3.1.0"
+      }
+    },
     "node_modules/http-errors": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/http-errors/-/http-errors-2.0.1.tgz",
@@ -6085,6 +6110,34 @@
         "node": ">= 14"
       }
     },
+    "node_modules/i18next": {
+      "version": "26.3.6",
+      "resolved": "https://registry.npmjs.org/i18next/-/i18next-26.3.6.tgz",
+      "integrity": "sha512-Bu5Z2nAXgfVyM8xvW3jk9EKRIuX37PudsrBViThNFx7CR7aaYTpP01cxNB/E4c4UUzTDiAZRstEhsRfPOL/8xA==",
+      "funding": [
+        {
+          "type": "individual",
+          "url": "https://www.locize.com/i18next"
+        },
+        {
+          "type": "individual",
+          "url": "https://www.i18next.com/how-to/faq#i18next-is-awesome.-how-can-i-support-the-project"
+        },
+        {
+          "type": "individual",
+          "url": "https://www.locize.com"
+        }
+      ],
+      "license": "MIT",
+      "peerDependencies": {
+        "typescript": "^5 || ^6 || ^7"
+      },
+      "peerDependenciesMeta": {
+        "typescript": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/ieee754": {
       "version": "1.2.1",
       "resolved": "https://registry.npmjs.org/ieee754/-/ieee754-1.2.1.tgz",
@@ -8347,6 +8400,33 @@
         "react": ">=17.0.0"
       }
     },
+    "node_modules/react-i18next": {
+      "version": "17.0.10",
+      "resolved": "https://registry.npmjs.org/react-i18next/-/react-i18next-17.0.10.tgz",
+      "integrity": "sha512-XneHftyYA774MJkkccSkZ5oKrUpCnXIPmxio3wemqrVzCRLWiGXOMbIzObrer03fNDEnm8g8R5yYls4HcE+esg==",
+      "license": "MIT",
+      "dependencies": {
+        "@babel/runtime": "^7.29.2",
+        "html-parse-stringify": "^3.0.1",
+        "use-sync-external-store": "^1.6.0"
+      },
+      "peerDependencies": {
+        "i18next": ">= 26.2.0",
+        "react": ">= 16.8.0",
+        "typescript": "^5 || ^6 || ^7"
+      },
+      "peerDependenciesMeta": {
+        "react-dom": {
+          "optional": true
+        },
+        "react-native": {
+          "optional": true
+        },
+        "typescript": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/react-is": {
       "version": "19.2.6",
       "resolved": "https://registry.npmjs.org/react-is/-/react-is-19.2.6.tgz",
@@ -8979,6 +9059,12 @@
         "node": "*"
       }
     },
+    "node_modules/rtl-detect": {
+      "version": "1.1.2",
+      "resolved": "https://registry.npmjs.org/rtl-detect/-/rtl-detect-1.1.2.tgz",
+      "integrity": "sha512-PGMBq03+TTG/p/cRB7HCLKJ1MgDIi07+QU1faSjiYRfmY5UsAttV9Hs08jDAHVwcOwmVLcSJkpwyfXszVjWfIQ==",
+      "license": "BSD-3-Clause"
+    },
     "node_modules/safe-buffer": {
       "version": "5.2.1",
       "resolved": "https://registry.npmjs.org/safe-buffer/-/safe-buffer-5.2.1.tgz",
@@ -10200,6 +10286,15 @@
       "integrity": "sha512-gQpnTgkubC6hQgdIcRdYGDSDc+SaujOdyesZQMv6JlfQee/9Mp0Qhnys6WxDWvQnL5WZdT7o2Ul187aSt0Rq+w==",
       "license": "MIT"
     },
+    "node_modules/void-elements": {
+      "version": "3.1.0",
+      "resolved": "https://registry.npmjs.org/void-elements/-/void-elements-3.1.0.tgz",
+      "integrity": "sha512-Dhxzh5HZuiHQhbvTW9AMetFfBHDMYpo23Uo9btPXgdYP+3T5S+p+jgNy7spra+veYhBP2dCSgxR/i2Y02h5/6w==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
     "node_modules/walker": {
       "version": "1.0.8",
       "resolved": "https://registry.npmjs.org/walker/-/walker-1.0.8.tgz",
diff --git a/package.json b/package.json
index 0e14da1..198ef27 100644
--- a/package.json
+++ b/package.json
@@ -26,14 +26,17 @@
     "expo-image-picker": "17.0.10",
     "expo-linking": "~8.0.11",
     "expo-local-authentication": "~17.0.8",
+    "expo-localization": "~17.0.9",
     "expo-notifications": "~0.32.16",
     "expo-router": "~6.0.22",
     "expo-secure-store": "~15.0.8",
     "expo-speech-recognition": "3.0.1",
     "expo-status-bar": "~3.0.9",
     "expo-store-review": "~9.0.9",
+    "i18next": "^26.3.6",
     "posthog-react-native": "^4.57.0",
     "react": "19.1.0",
+    "react-i18next": "^17.0.10",
     "react-native": "0.81.5",
     "react-native-gesture-handler": "~2.28.0",
     "react-native-marked": "8.0.0",
diff --git a/src/lib/i18n/config.ts b/src/lib/i18n/config.ts
new file mode 100644
index 0000000..b02b0cf
--- /dev/null
+++ b/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
diff --git a/src/lib/i18n/en.json b/src/lib/i18n/en.json
new file mode 100644
index 0000000..07210e7
--- /dev/null
+++ b/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."
+    }
+  }
+}
diff --git a/src/lib/i18n/locale-resolve.test.ts b/src/lib/i18n/locale-resolve.test.ts
new file mode 100644
index 0000000..075e05e
--- /dev/null
+++ b/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)
+})
diff --git a/src/lib/i18n/locale-resolve.ts b/src/lib/i18n/locale-resolve.ts
new file mode 100644
index 0000000..2ace725
--- /dev/null
+++ b/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
+}
diff --git a/src/lib/i18n/zh-Hans.json b/src/lib/i18n/zh-Hans.json
new file mode 100644
index 0000000..2604715
--- /dev/null
+++ b/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。"
+    }
+  }
+}
diff --git a/src/stores/settings.ts b/src/stores/settings.ts
index 5ab86d9..8a57b3a 100644
--- a/src/stores/settings.ts
+++ b/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 })
+  },
 }))