Implement basic localization

[TimJentzsch]

Localization gets harder to add the more UI we have, so it's best to start this early.

There aren't really mature localization options available for Bevy yet, but we can start out with bevy_mod_localization (using a git dependency).

It will probably be rather boilerplate-y at the start, but once the use-cases are more clear we can also improve bevy_mod_localization to make common patterns easier.