diff --git a/Memory/ARCHITECTURE.md b/Memory/ARCHITECTURE.md
index 079c5d3c..cacc3820 100644
--- a/Memory/ARCHITECTURE.md
+++ b/Memory/ARCHITECTURE.md
@@ -80,6 +80,8 @@ App appearance is a global UserDefaults preference applied by the macOS app
 delegate through `NSApplication.appearance`; following System clears the override.
 Opening at login reflects `SMAppService` on every Settings visit rather than a
 mirrored preference. A quiet login hides the app without suppressing its window.
+macOS owns an explicit in-app language preference. Mobile delegates language selection
+to Apple's per-app Settings and never stores or injects its own locale override.
 
 ## Remote sessions, terminal, and files
 
diff --git a/TFI/App/MobileLanguagePreferenceMigration.swift b/TFI/App/MobileLanguagePreferenceMigration.swift
new file mode 100644
index 00000000..dcd4c75b
--- /dev/null
+++ b/TFI/App/MobileLanguagePreferenceMigration.swift
@@ -0,0 +1,10 @@
+import Foundation
+import TermindApplication
+
+nonisolated enum MobileLanguagePreferenceMigration {
+    static func run(in defaults: UserDefaults = .standard) {
+        guard defaults.object(forKey: SettingsKey.appLanguage) != nil else { return }
+        defaults.removeObject(forKey: SettingsKey.appLanguage)
+        defaults.removeObject(forKey: "AppleLanguages")
+    }
+}
diff --git a/TFI/App/TermindIOSApp.swift b/TFI/App/TermindIOSApp.swift
index aa3fcea1..8349b1ba 100644
--- a/TFI/App/TermindIOSApp.swift
+++ b/TFI/App/TermindIOSApp.swift
@@ -6,10 +6,10 @@ import TermindApplication
 struct TermindIOSApp: App {
     @UIApplicationDelegateAdaptor(MobileAppDelegate.self) private var appDelegate
     @AppStorage("mobile.appearance") private var appearance = "system"
-    private let launchLanguage = AppLanguage.stored()
     private let startup: Result<MobileAppModel, any Error>
 
     init() {
+        MobileLanguagePreferenceMigration.run()
         startup = Result {
             let isTesting = ProcessInfo.processInfo.environment["XCTestConfigurationFilePath"] != nil
             return try MobileAppModel(dependencies: isTesting ? .testing() : .live())
@@ -23,7 +23,6 @@ struct TermindIOSApp: App {
                 MobileQuickActionSceneRoot(app: app)
                     .modelContainer(app.container)
                     .environment(\.mobileInterfaceIdiom, MobileInterfaceIdiom.current)
-                    .environment(\.locale, launchLanguage.locale)
                     .preferredColorScheme(appearance == "dark" ? .dark : appearance == "light" ? .light : nil)
             case .failure:
                 ContentUnavailableView(
diff --git a/TFI/Features/Settings/MobileAppSettingsView.swift b/TFI/Features/Settings/MobileAppSettingsView.swift
index eeb9bad7..f3726321 100644
--- a/TFI/Features/Settings/MobileAppSettingsView.swift
+++ b/TFI/Features/Settings/MobileAppSettingsView.swift
@@ -5,21 +5,6 @@ struct MobileAppSettingsView: View {
     var appReview: AppReviewController?
     var reviewScene: MobileSceneModel?
     @AppStorage("mobile.appearance") private var appearance = "system"
-    @AppStorage(SettingsKey.appLanguage) private var language = AppLanguage.system.rawValue
-    @State private var launchLanguage = AppLanguage.stored()
-
-    private var selectedLanguage: AppLanguage {
-        AppLanguage(rawValue: language) ?? AppLanguage.stored()
-    }
-
-    private var languageSelection: Binding<AppLanguage> {
-        Binding(
-            get: { selectedLanguage },
-            set: { language in
-                self.language = language.rawValue
-                AppLanguage.store(language)
-            })
-    }
 
     var body: some View {
         Form {
@@ -29,20 +14,6 @@ struct MobileAppSettingsView: View {
                     Text("Light").tag("light")
                     Text("Dark").tag("dark")
                 }
-                Picker("Language", selection: languageSelection) {
-                    ForEach(AppLanguage.allCases) {
-                        Text(
-                            $0.displayName(
-                                deviceRegion: Locale.autoupdatingCurrent.region?.identifier,
-                                canDisplayTaiwanFlag: DeviceCensorship.canDisplayTaiwanFlag
-                            )
-                        ).tag($0)
-                    }
-                }
-                if selectedLanguage != launchLanguage {
-                    Text("The new language will be used after Termind relaunches.").font(.caption).foregroundStyle(
-                        .secondary)
-                }
             }
             Section("Version") {
                 LabeledContent(
diff --git a/TFITests/MobileLanguagePreferenceMigrationTests.swift b/TFITests/MobileLanguagePreferenceMigrationTests.swift
new file mode 100644
index 00000000..947afe78
--- /dev/null
+++ b/TFITests/MobileLanguagePreferenceMigrationTests.swift
@@ -0,0 +1,37 @@
+import Foundation
+import TermindApplication
+import Testing
+
+@testable import TermindIOS
+
+struct MobileLanguagePreferenceMigrationTests {
+    @Test func removesLegacyInAppLanguagePreferences() throws {
+        let (defaults, suiteName) = try makeDefaults()
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+        defaults.set(AppLanguage.simplifiedChinese.rawValue, forKey: SettingsKey.appLanguage)
+        defaults.set([AppLanguage.simplifiedChinese.rawValue], forKey: "AppleLanguages")
+
+        MobileLanguagePreferenceMigration.run(in: defaults)
+
+        let domain = defaults.persistentDomain(forName: suiteName)
+        #expect(domain?[SettingsKey.appLanguage] == nil)
+        #expect(domain?["AppleLanguages"] == nil)
+    }
+
+    @Test func preservesSystemManagedPerAppLanguageWithoutLegacyMarker() throws {
+        let (defaults, suiteName) = try makeDefaults()
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+        defaults.set([AppLanguage.traditionalChinese.rawValue], forKey: "AppleLanguages")
+
+        MobileLanguagePreferenceMigration.run(in: defaults)
+
+        #expect(defaults.stringArray(forKey: "AppleLanguages") == [AppLanguage.traditionalChinese.rawValue])
+    }
+
+    private func makeDefaults() throws -> (UserDefaults, String) {
+        let suiteName = "MobileLanguagePreferenceMigrationTests.\(UUID().uuidString)"
+        let defaults = try #require(UserDefaults(suiteName: suiteName))
+        defaults.removePersistentDomain(forName: suiteName)
+        return (defaults, suiteName)
+    }
+}