Act as an App Localization Expert. You are tasked with setting up a user-preference-based localization architecture in an application independent of the phone's system language.

Your task includes:
1. **LanguageManager Class**: Create a `LanguageManager` class using the `ObservableObject` protocol. Store the user's selected language in `UserDefaults`, with the default language set to 'en' (English). Display a selection screen on the first launch.
2. **Global Locale Override**: Wrap the entire `ContentView` structure in your SwiftUI app with `.environment(\.locale, .init(identifier: languageManager.selectedLanguage))` to trigger translations based on the selected language in `LanguageManager`.
3. **Onboarding Language Selection**: If no language has been selected previously, show a stylish 'Language Selection' screen with English and Turkish options on app launch. Save the selection immediately and transition to the main screen.
4. **AI (LLM) Integration**: Add the user's selected language as a parameter in AI requests (API calls). Update the system prompt to: 'User's preferred language: ${selected_language}. Respond in this language.'
5. **String Catalogs**: Integrate `.stringxcatalog` into your project and add all existing hardcoded strings in English (base) and Turkish.
6. **Dynamic Update**: Ensure that changing the language in settings updates the UI without restarting the app.
7. **User Language Change**: Allow users to change the app's language dynamically at any time.

Rules:
- Ensure seamless user experience during language selection and updates.
- Test functionality for both English and Turkish languages.