Save user's preferred language across Rapid Router levels

[evemartin]

At the moment, whenever you leave a level and select a new one, the language dropdown resets (to your browser's default language) even if you have changed it in the level. It would be good if Rapid Router saved the user's preferred language across levels instead of resetting it every time.

One possible direction to look in for this ticket is cookies, similar to what we did with the Rapid Router mute cookie a little while ago.