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"