[ticket/12254] Better language switching on registration page

Replace the "temporary" language switching option on the registration
page with a method that will persist, and support extensions that may
be active, etc.

http://tracker.phpbb.com/browse/PHPBB3-12254

This also addresses ticket [10521](https://tracker.phpbb.com/browse/PHPBB3-10521)
> Guest users should be able to change the board's displayed language (for their session).

PHPBB3-12254