Overhaul language selector
Now it's an OverlaySheet which is way less hacky than our custom ComboBox reimplementation, and comes with more fringe benefits. For one, native language names are used if available to us. Another is the selected language is now presented in the center of the selection, so you don't lose the selection. KItemModels is used instead of us having another reimplementation of it.
A separate backportable fix that is less enthusiastic that changes the languages to their native names will be made available for 23.08 soon.