feat(i18n): data-driven language picker + locale config

Make the supported-language list a single source of truth so community
translations show up with no code change: add res/xml/locales_config.xml
(en, de) and reference it via android:localeConfig, which also surfaces the
per-app language entry in Android 13+ system settings.

Rewrite AppLanguage to parse locales_config.xml for the supported BCP-47
tags and expose currentTag/apply/displayName (autonyms), dropping the
hardcoded LanguagePref enum; the Settings picker is now built from that list.
Remove the now-unused settings_language_german/english strings.

Adding a language is now: drop in values-<tag>/strings.xml and add one
<locale> line.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/AndroidManifest.xml

@@ -25,6 +25,7 @@
         android:fullBackupContent="@xml/backup_rules"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
+        android:localeConfig="@xml/locales_config"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
         android:theme="@style/Theme.Calendula"

app/src/main/java/de/jeanlucmakiola/calendula/ui/settings/AppLanguage.kt

@@ -1,36 +1,78 @@
 package de.jeanlucmakiola.calendula.ui.settings
 
+import android.content.Context
 import androidx.appcompat.app.AppCompatDelegate
 import androidx.core.os.LocaleListCompat
+import de.jeanlucmakiola.calendula.R
+import org.xmlpull.v1.XmlPullParser
+import java.util.Locale
 
-/** UI-facing language choice. AUTO follows the system languages. */
-enum class LanguagePref { AUTO, GERMAN, ENGLISH }
+private const val ANDROID_NS = "http://schemas.android.com/apk/res/android"
 
 /**
- * Per-app language via AppCompatDelegate. On API 33+ this delegates to the
- * platform per-app-languages API; below that the appcompat backport persists
- * the choice itself (manifest `autoStoreLocales` service), so we don't mirror
- * it in DataStore. Setting a locale recreates the activity, which re-reads the
- * current value for the dropdown.
+ * Per-app language via AppCompatDelegate, driven by res/xml/locales_config.xml.
+ *
+ * That file is the single source of truth for which languages we ship: dropping
+ * in a values-<tag> translation and adding a matching `<locale>` entry makes the
+ * language show up here and in the system per-app-language settings, with no
+ * other code change. The system-default choice is represented as `null`.
+ *
+ * On API 33+ this delegates to the platform per-app-languages API; below that
+ * the appcompat backport persists the choice itself (manifest `autoStoreLocales`
+ * service), so we don't mirror it in DataStore. Setting a locale recreates the
+ * activity, which re-reads the current value for the picker.
  */
 object AppLanguage {
 
-    fun current(): LanguagePref {
-        val locales = AppCompatDelegate.getApplicationLocales()
-        if (locales.isEmpty) return LanguagePref.AUTO
-        return when (locales[0]?.language) {
-            "de" -> LanguagePref.GERMAN
-            "en" -> LanguagePref.ENGLISH
-            else -> LanguagePref.AUTO
+    /**
+     * The BCP-47 tags the app ships translations for, in declaration order, as
+     * listed in locales_config.xml. Returns whatever could be parsed; a missing
+     * or malformed config yields an empty list (the picker then offers only the
+     * system-default entry rather than crashing).
+     */
+    fun supportedTags(context: Context): List<String> {
+        val tags = mutableListOf<String>()
+        val parser = context.resources.getXml(R.xml.locales_config)
+        try {
+            var event = parser.eventType
+            while (event != XmlPullParser.END_DOCUMENT) {
+                if (event == XmlPullParser.START_TAG && parser.name == "locale") {
+                    parser.getAttributeValue(ANDROID_NS, "name")?.let(tags::add)
+                }
+                event = parser.next()
+            }
+        } catch (_: Exception) {
+            // Fall back to whatever was parsed before the failure.
+        } finally {
+            parser.close()
         }
+        return tags
     }
 
-    fun apply(pref: LanguagePref) {
-        val locales = when (pref) {
-            LanguagePref.AUTO -> LocaleListCompat.getEmptyLocaleList()
-            LanguagePref.GERMAN -> LocaleListCompat.forLanguageTags("de")
-            LanguagePref.ENGLISH -> LocaleListCompat.forLanguageTags("en")
+    /** The applied app language as a BCP-47 tag, or `null` when following the system. */
+    fun currentTag(): String? {
+        val locales = AppCompatDelegate.getApplicationLocales()
+        return if (locales.isEmpty) null else locales[0]?.toLanguageTag()
+    }
+
+    /** Apply a BCP-47 tag, or `null` to follow the system languages. */
+    fun apply(tag: String?) {
+        val locales = if (tag == null) {
+            LocaleListCompat.getEmptyLocaleList()
+        } else {
+            LocaleListCompat.forLanguageTags(tag)
         }
         AppCompatDelegate.setApplicationLocales(locales)
     }
+
+    /**
+     * The autonym for a tag — the language's own name in its own script, e.g.
+     * "Deutsch", "English", "Français" — so users find their language regardless
+     * of the current UI language. Capitalised per the language's own rules.
+     */
+    fun displayName(tag: String): String {
+        val locale = Locale.forLanguageTag(tag)
+        return locale.getDisplayName(locale)
+            .replaceFirstChar { if (it.isLowerCase()) it.titlecase(locale) else it.toString() }
+    }
 }

app/src/main/java/de/jeanlucmakiola/calendula/ui/settings/SettingsScreen.kt

@@ -188,11 +188,15 @@ private fun SettingsHub(
 
 @Composable
 private fun LanguageRow(position: Position) {
+    val context = LocalContext.current
     // Setting a locale recreates the activity; mirror the choice locally so the
     // row updates instantly even before the recreation lands.
-    var current by remember { mutableStateOf(AppLanguage.current()) }
+    var current by remember { mutableStateOf(AppLanguage.currentTag()) }
     var showDialog by remember { mutableStateOf(false) }
 
+    // null = follow the system; the rest are BCP-47 tags from locales_config.xml.
+    val options = remember { listOf<String?>(null) + AppLanguage.supportedTags(context) }
+
     GroupedRow(
         title = stringResource(R.string.settings_language),
         summary = languageLabel(current),
@@ -204,7 +208,7 @@ private fun LanguageRow(position: Position) {
     if (showDialog) {
         OptionPickerDialog(
             title = stringResource(R.string.settings_language),
-            options = LanguagePref.entries,
+            options = options,
             selected = current,
             label = { languageLabel(it) },
             onSelect = {
@@ -598,10 +602,5 @@ private fun weekStartLabel(pref: WeekStartPref): String = stringResource(
 )
 
 @Composable
-private fun languageLabel(pref: LanguagePref): String = stringResource(
-    when (pref) {
-        LanguagePref.AUTO -> R.string.settings_language_auto
-        LanguagePref.GERMAN -> R.string.settings_language_german
-        LanguagePref.ENGLISH -> R.string.settings_language_english
-    },
-)
+private fun languageLabel(tag: String?): String =
+    if (tag == null) stringResource(R.string.settings_language_auto) else AppLanguage.displayName(tag)

app/src/main/res/values-de/strings.xml

@@ -254,8 +254,6 @@
     <string name="settings_section_language">Sprache</string>
     <string name="settings_language">App-Sprache</string>
     <string name="settings_language_auto">Systemstandard</string>
-    <string name="settings_language_german">Deutsch</string>
-    <string name="settings_language_english">English</string>
     <!-- Hub category subtitles -->
     <string name="settings_appearance_subtitle">Design, dynamische Farben, Wochenstart</string>
     <string name="settings_event_form_subtitle">Standardfelder für neue Termine</string>

app/src/main/res/values/strings.xml

@@ -251,8 +251,6 @@
     <string name="settings_section_language">Language</string>
     <string name="settings_language">App language</string>
     <string name="settings_language_auto">System default</string>
-    <string name="settings_language_german">Deutsch</string>
-    <string name="settings_language_english">English</string>
     <!-- Hub category subtitles -->
     <string name="settings_appearance_subtitle">Theme, dynamic colour, week start</string>
     <string name="settings_event_form_subtitle">Default fields for new events</string>

app/src/main/res/xml/locales_config.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+    The languages Calendula ships translations for. This is the single source of
+    truth: each entry must have a matching res/values-<tag>/strings.xml, and is
+    surfaced automatically in both the in-app language picker (parsed at runtime
+    by AppLanguage) and the system per-app language settings (Android 13+, via
+    android:localeConfig in the manifest). To add a community translation, drop
+    in the values-<tag> folder and add one <locale> line here.
+-->
+<locale-config xmlns:android="http://schemas.android.com/apk/res/android">
+    <locale android:name="en" />
+    <locale android:name="de" />
+</locale-config>