language: Wait for locale generation before going forward
If a user chooses a locale that's not generated yet, selecting it will trigger locale generation. This can take a while, but happens in the background and isn't indicated in any way. When the user chooses another locale while the previous one is still generating, the new selection gets lost. Also, a newly chosen language is not available yet for the g-i-s to switch its UI to. The UI should tell the user to wait until the generation is done.