Add language selector in Letterbox demo

[defagos]

To make it easier to test layouts in all supported languages, the demo should provide a language selector. No need to allow switching to be made at runtime, we can just kill the app and let the new language setting be applied after restarting it.

This is much faster than changing the language in system settings. We also don’t need to localize the demo itself (this is just for testing the player localization).