docs(mobile-client): add localization guide

- Explain how to add new translation strings and support for new languages
- Describe the structure and purpose of localization files (.arb)
- Provide step-by-step instructions for adding translations and new languages
- Include information on generating Dart code and using localized strings in UI

Author: fulleni

src/content/docs/mobile-client/guides/localization.mdx

@@ -0,0 +1,79 @@
+---
+title: 'Guide: Localization'
+description: A guide to adding new languages and translations to the mobile client.
+---
+import { Steps, Aside } from '@astrojs/starlight/components';
+
+The mobile client is fully internationalized using Flutter's built-in localization support. This guide explains how to add new translation strings and support for new languages.
+
+### Localization Files
+
+All translation strings are stored in Application Resource Bundle (`.arb`) files located in the `lib/l10n/arb/` directory.
+
+-   `app_en.arb`: Contains the English translations. This is the template file.
+-   `app_ar.arb`: Contains the Arabic translations.
+
+Each entry in an `.arb` file consists of a key, the translated string, and an optional description.
+
+```json title="lib/l10n/arb/app_en.arb"
+{
+    "@@locale": "en",
+    "accountPageTitle": "Account",
+    "@accountPageTitle": {
+        "description": "Title for the account page"
+    }
+}
+```
+
+### How to Add a New Translation String
+
+<Steps>
+1.  **Add the Key to the English Template**
+
+    Open `lib/l10n/arb/app_en.arb` and add a new entry for your string. Provide a clear key and a helpful description.
+
+    ```json
+    "myNewFeatureTitle": "My New Feature",
+    "@myNewFeatureTitle": {
+        "description": "Title for the new feature page"
+    }
+    ```
+
+2.  **Add Translations to Other Languages**
+
+    Open the `.arb` file for each supported language (e.g., `app_ar.arb`) and add the corresponding translated string for the new key.
+
+    ```json
+    "myNewFeatureTitle": "ميزتي الجديدة",
+    ```
+
+3.  **Generate Dart Code**
+
+    Flutter's localization tool automatically generates Dart code from your `.arb` files. This process is usually triggered automatically when you save an `.arb` file if you are using a modern IDE with the Flutter extension.
+
+    If you need to run it manually, execute the following command in your terminal:
+    ```bash
+    flutter gen-l10n
+    ```
+
+4.  **Use the String in Your UI**
+
+    You can now access your new string in any widget that has a `BuildContext` using the generated `AppLocalizations` class.
+
+    ```dart
+    import 'package:flutter_news_app_mobile_client_full_source_code/l10n/l10n.dart';
+
+    // ... inside a build method
+    Text(context.l10n.myNewFeatureTitle);
+    ```
+
+</Steps>
+
+### How to Add a New Language
+
+To add support for a new language, such as French:
+1.  Create a new file: `lib/l10n/arb/app_fr.arb`.
+2.  Set the locale at the top of the file: `"@@locale": "fr"`.
+3.  Copy all the keys from `app_en.arb` and provide the French translations.
+4.  Run `flutter gen-l10n` to update the generated code.
+5.  Finally, add the new language to the list of supported languages in `lib/app/view/app.dart`.