[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. The selected language will be stored in a session_lang
column in the DB. This PR keeps the concept originally put in place
intact...it only applies to guest's sessions, and does not implement a
"quick-language-menu" throughout the entire board.

http://tracker.phpbb.com/browse/PHPBB3-12254

PHPBB3-12254