Well, I implemented this feature within the session.php of phpBB3.0-RC4. My intension was to let the user decide the language before entering the board, so I rather implemented this using a GET-variable and a cookie. As soon as the user is logged in, his/her settings are overwriting my hack.
Basically I am not a phpBB hacker, but it's working so far, except for one little bug I couldn't fix, yet: The localized images are still appearing in the default language.
I have no idea for what reason it doesn't display the correct images for "New Topic", "Reply", etc.. Maybe some more experienced phpBB coders could help here.