[auth][l10n]: Add Bulgarian translation (#3402)

## Description

## Tests

auth/lib/locale.dart

@@ -5,6 +5,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 // Add more language to the list only when at least 90% of the strings are
 // translated in the corresponding language.
 const List<Locale> appSupportedLocales = <Locale>[
+  Locale('bg'),
   Locale('de'),
   Locale('en'),
   Locale('es', 'ES'),

auth/lib/ui/settings/language_picker.dart

@@ -126,6 +126,8 @@ class _ItemsWidgetState extends State<ItemsWidget> {
     switch (locale.languageCode) {
       case 'en':
         return 'English';
+      case 'bg':
+        return 'Български';
       case 'es':
         switch (locale.countryCode) {
           case 'ES':