feat: in-app display language switcher (Settings → Appearance) (#604)

Add a Language selector to Settings → Appearance that overrides the app
display language independent of the device locale. Persists a BCP-47 code
in ServerStoreState.appLanguage; MainActivity.attachBaseContext applies it
via a ContextWrapper (works on minSdk 26, no AppCompat dependency needed
since the Activity is a plain ComponentActivity). Selecting a language
recreates the activity so resources re-resolve immediately.

Includes System default / English / 한국어 options, plus a unit test for
the locale-code resolver.

Note: the new string keys (settings_item_language, language_system,
language_english, language_korean) should be backfilled into values-ko
once PR #602 lands.

Co-authored-by: hy4ri <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/MainActivity.kt

@@ -1,5 +1,6 @@
 package com.m57.hermescontrol
 
+import android.content.Context
 import android.content.Intent
 import android.os.Bundle
 import androidx.activity.ComponentActivity
@@ -16,11 +17,24 @@ import androidx.compose.ui.Modifier
 import com.m57.hermescontrol.data.local.AuthManager
 import com.m57.hermescontrol.notification.NotificationReplyReceiver
 import com.m57.hermescontrol.theme.HermesControlTheme
+import com.m57.hermescontrol.util.LocaleContextWrapper
 
 class MainActivity : ComponentActivity() {
     // Observable state so both onCreate and onNewIntent updates flow to Compose
     private var notificationSessionId by mutableStateOf<String?>(null)
 
+    /**
+     * Apply the user-selected display language before any view is inflated.
+     * Reads the persisted code from [AuthManager]; an uninitialized store
+     * (shouldn't happen here, but guarded) falls back to the device locale.
+     */
+    override fun attachBaseContext(base: Context) {
+        val code =
+            runCatching { AuthManager.getAppLanguage() }
+                .getOrDefault(LocaleContextWrapper.SYSTEM_LANGUAGE)
+        super.attachBaseContext(LocaleContextWrapper.wrapWithCode(base, code))
+    }
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
 

app/src/main/java/com/m57/hermescontrol/data/config/ServerStoreState.kt

@@ -23,4 +23,7 @@ data class ServerStoreState(
     val wsAuthParam: String = "token",
     val typingEffectEnabled: Boolean = true,
     val typingEffectDelayMs: Int = 30,
+    // App display language. "system" = follow device locale; otherwise a BCP-47
+    // language code such as "en" or "ko". Applied via ContextWrapper in MainActivity.
+    val appLanguage: String = "system",
 )

app/src/main/java/com/m57/hermescontrol/data/local/AuthManager.kt

@@ -498,6 +498,15 @@ object AuthManager {
         serverStore.update { it.copy(typingEffectEnabled = enabled) }
     }
 
+    // ── App display language ───────────────────────────────────────────
+    // "system" follows the device locale; any other value is a BCP-47 code.
+
+    fun getAppLanguage(): String = serverStore.getLatestState().appLanguage
+
+    fun setAppLanguage(code: String) {
+        serverStore.update { it.copy(appLanguage = code) }
+    }
+
     fun getTypingEffectDelayMs(): Int = serverStore.getLatestState().typingEffectDelayMs
 
     fun setTypingEffectDelayMs(delayMs: Int) {

app/src/main/java/com/m57/hermescontrol/ui/settings/SettingsScreen.kt

@@ -163,6 +163,8 @@ fun SettingsScreen(
                             onUseDynamicColorsChange = viewModel::onUseDynamicColorsChange,
                             themePreset = state.themePreset,
                             onThemePresetChange = viewModel::onThemePresetChange,
+                            appLanguage = state.appLanguage,
+                            onAppLanguageChange = viewModel::onAppLanguageChange,
                         )
 
                         ChatSection(

app/src/main/java/com/m57/hermescontrol/ui/settings/SettingsViewModel.kt

@@ -39,6 +39,7 @@ data class SettingsUiState(
     val selectedProfileId: String? = null,
     val renameProfileName: String = "",
     val bottomNavDisplayMode: BottomNavDisplayMode = BottomNavDisplayMode.ICON_AND_TEXT,
+    val appLanguage: String = "system",
     // Profile add/edit dialog
     val showProfileDialog: Boolean = false,
     val editingProfileId: String? = null,
@@ -79,6 +80,7 @@ class SettingsViewModel(
         val typingEffectDelayMs = AuthManager.getTypingEffectDelayMs()
         val profiles = AuthManager.getConnectionProfiles()
         val bottomNavDisplayMode = AuthManager.getBottomNavDisplayMode()
+        val appLanguage = AuthManager.getAppLanguage()
         val renameProfileName =
             profiles.firstOrNull { p -> p.id == selectedId }?.name ?: ""
         _uiState.update {
@@ -97,6 +99,7 @@ class SettingsViewModel(
                 selectedProfileId = selectedId,
                 renameProfileName = renameProfileName,
                 bottomNavDisplayMode = bottomNavDisplayMode,
+                appLanguage = appLanguage,
             )
         }
     }
@@ -296,6 +299,11 @@ class SettingsViewModel(
         AuthManager.setThemePreset(preset)
     }
 
+    fun onAppLanguageChange(code: String) {
+        _uiState.update { it.copy(appLanguage = code, isSaved = false) }
+        AuthManager.setAppLanguage(code)
+    }
+
     fun onBottomNavDisplayModeChange(mode: BottomNavDisplayMode) {
         _uiState.update { it.copy(bottomNavDisplayMode = mode, isSaved = false) }
         AuthManager.setBottomNavDisplayMode(mode)

app/src/main/java/com/m57/hermescontrol/ui/settings/components/AppearanceSection.kt

@@ -1,5 +1,6 @@
 package com.m57.hermescontrol.ui.settings.components
 
+import androidx.activity.compose.LocalActivity
 import androidx.compose.animation.AnimatedVisibility
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
@@ -43,6 +44,7 @@ import com.m57.hermescontrol.theme.BottomNavDisplayMode
 import com.m57.hermescontrol.theme.ThemePreference
 import com.m57.hermescontrol.theme.ThemePreset
 import com.m57.hermescontrol.ui.settings.SectionCard
+import com.m57.hermescontrol.util.LocaleContextWrapper
 
 @Composable
 internal fun AppearanceSection(
@@ -52,6 +54,8 @@ internal fun AppearanceSection(
     onUseDynamicColorsChange: (Boolean) -> Unit,
     themePreset: ThemePreset,
     onThemePresetChange: (ThemePreset) -> Unit,
+    appLanguage: String,
+    onAppLanguageChange: (String) -> Unit,
 ) {
     SectionCard(title = stringResource(R.string.settings_sec_appearance)) {
         Text(
@@ -193,6 +197,42 @@ internal fun AppearanceSection(
                 }
             }
         }
+
+        Spacer(modifier = Modifier.height(16.dp))
+
+        Text(
+            text = stringResource(R.string.settings_item_language),
+            style = MaterialTheme.typography.bodyLarge,
+        )
+        Spacer(modifier = Modifier.height(8.dp))
+        val languageOptions =
+            listOf(
+                LocaleContextWrapper.SYSTEM_LANGUAGE to stringResource(R.string.language_system),
+                "en" to stringResource(R.string.language_english),
+                "ko" to stringResource(R.string.language_korean),
+            )
+        SingleChoiceSegmentedButtonRow(modifier = Modifier.fillMaxWidth()) {
+            val activity = LocalActivity.current
+            languageOptions.forEachIndexed { index, (code, label) ->
+                SegmentedButton(
+                    selected = appLanguage == code,
+                    onClick = {
+                        onAppLanguageChange(code)
+                        // MainActivity is a plain ComponentActivity (not
+                        // AppCompatActivity), so the locale only takes effect
+                        // after the activity is recreated.
+                        activity?.recreate()
+                    },
+                    shape =
+                        SegmentedButtonDefaults.itemShape(
+                            index = index,
+                            count = languageOptions.size,
+                        ),
+                ) {
+                    Text(label)
+                }
+            }
+        }
     }
 }
 

app/src/main/java/com/m57/hermescontrol/util/LocaleContextWrapper.kt

@@ -0,0 +1,73 @@
+package com.m57.hermescontrol.util
+
+import android.content.Context
+import android.content.ContextWrapper
+import android.os.Build
+import java.util.Locale
+
+/**
+ * Static helpers for applying a user-selected in-app display language.
+ *
+ * Hermes Mobile's [com.m57.hermescontrol.MainActivity] extends the plain
+ * `ComponentActivity` (not `AppCompatActivity`), so we apply the locale through
+ * `attachBaseContext` + [ContextWrapper.wrap] rather than relying on
+ * AppCompatDelegate. Works on minSdk 26.
+ */
+object LocaleContextWrapper {
+    const val SYSTEM_LANGUAGE = "system"
+
+    /**
+     * Resolve a language code ("system", "en", "ko", …) into a [Locale].
+     *
+     * "system" returns the device's default locale (so passing it through
+     * [wrap] is effectively a no-op). Codes containing a region separator
+     * ("zh-rCN", "pt-BR") are split into language + country.
+     */
+    fun localeForCode(code: String): Locale =
+        when {
+            code.isEmpty() || code == SYSTEM_LANGUAGE -> Locale.getDefault()
+            code.contains("-r", ignoreCase = true) -> {
+                val parts = code.split("-r", ignoreCase = true)
+                Locale(parts[0], parts.getOrElse(1) { "" })
+            }
+            code.contains("-") -> {
+                val parts = code.split("-")
+                Locale(parts[0], parts.getOrElse(1) { "" })
+            }
+            code.contains("_") -> {
+                val parts = code.split("_")
+                Locale(parts[0], parts.getOrElse(1) { "" })
+            }
+            else -> Locale(code)
+        }
+
+    /**
+     * Wrap [base] so the configuration carries [locale].
+     *
+     * On API 33+ we use [Context.createConfigurationContext] which is the only
+     * reliable path once the deprecated `Configuration.locale` / `setLocale`
+     * APIs were removed from the public surface.
+     */
+    fun wrap(
+        base: Context,
+        locale: Locale,
+    ): Context {
+        val config = base.resources.configuration
+        val newConfig =
+            android.content.res.Configuration(config).apply {
+                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+                    setLocales(android.os.LocaleList(locale))
+                } else {
+                    @Suppress("DEPRECATION")
+                    setLocale(locale)
+                }
+            }
+        return base.createConfigurationContext(newConfig)
+    }
+
+    /** Convenience: wrap [base] using a stored language code. */
+    fun wrapWithCode(
+        base: Context,
+        code: String,
+    ): Context = wrap(base, localeForCode(code))
+}