Adds a 'language' dropdown field to the system configuration
Created by: fonfon
- The language choice is stored within the session, not persistently (in kvstore)
- This removes confirmation messages if nothing was changed
Maybe there is a better way to detect the plinth / locale directory than this: https://github.com/fonfon/Plinth/blob/language-detect/plinth/main.py#L218
If a user chooses a certain language his setting survives login/logout with the same browser, but won't survive using another browser or device. Thus we should use kvstore in the long run to store user language choices.