feat(ui): add LanguageDropdownFormField widget

- Create a reusable dropdown form field for selecting a language
- Add support for custom languages list, initial value, and label text
- Implement localization using AppLocalizationsX

Author: fulleni

language_dropdown_form_field.dart

@@ -0,0 +1,47 @@
+import 'package:core/core.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_news_app_web_dashboard_full_source_code/l10n/l10n.dart';
+
+/// A reusable dropdown form field for selecting a language.
+class LanguageDropdownFormField extends StatelessWidget {
+  /// {@macro language_dropdown_form_field}
+  const LanguageDropdownFormField({
+    required this.languages,
+    required this.onChanged,
+    this.initialValue,
+    this.labelText,
+    super.key,
+  });
+
+  /// The list of languages to display in the dropdown.
+  final List<Language> languages;
+
+  /// The currently selected language.
+  final Language? initialValue;
+
+  /// The callback that is called when the user selects a language.
+  final ValueChanged<Language?> onChanged;
+
+  /// The text to display as the label for the form field.
+  final String? labelText;
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizationsX(context).l10n;
+
+    return DropdownButtonFormField<Language?>(
+      value: initialValue,
+      decoration: InputDecoration(
+        labelText: labelText ?? l10n.language,
+        border: const OutlineInputBorder(),
+      ),
+      items: [
+        DropdownMenuItem(value: null, child: Text(l10n.none)),
+        ...languages.map(
+          (l) => DropdownMenuItem(value: l, child: Text(l.name)),
+        ),
+      ],
+      onChanged: onChanged,
+    );
+  }
+}