[Feature request]: Add support for setting locale/language in Google Maps navigation UI

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Use case

When using google_navigation_flutter, the language of the navigation UI (e.g., turn-by-turn voice guidance, on-screen text) defaults to English. However, many apps need to offer navigation in a different language than the system language — for example:

- Multilingual apps where users select a preferred app language
- Touristic or travel apps that provide guidance in the user’s native language
- Accessibility scenarios where voice directions in a specific language are required

Currently, there is no option in the Flutter plugin to override the locale passed to Google Maps navigation, which limits flexibility and breaks language consistency within the app.

### Proposal

Add a locale or languageCode parameter to the GoogleMapsNavigator. This value would be passed to the underlying navigation engine (iOS/Android) to control spoken instructions and on-screen text.