Draft: Update panel elements when language is changed

gnome-initial-setup has this wart where a user picks a language in the language list and the panel doesn't update right away.

Obviously completely changing the locale in a running session isn't easy or something we can really do, but maybe we can at least get the main bits of the shell changed to improve the experience a bit.

This is Draft because it's just the start of a sketch, only changing the parts gnome-initial-setup can see. We'd probably want to add code to rebuild a few other part of the UI before landing this if we decide to go this route.