The "language" only defines the user interface, it will not effect any posts. The language is setable per user and that makes most sense. If a user is more comfortable using language A then he probably want to have his user interface everywhere on the board in that language.
That makes sense but suppose a german user wants to register on an english board, then until he registers he cannot switch his settings to german? The registration system should be in their language. Is there a way on my registration page I can add a link to their language? What would the url for that switch be? Thanks!