Would it be possible to use a hidden forums option? You could use the language preference the user decides on as a classification. This could be used kind of like your user/moderator/administrator status is used to determine which forums you can look at if all forums were marked as belonging to a specific language. Then, even though the database is UTF and completely multilingual, each individual user could only see the forums that match their language parameter. The only way you could access the other content would be to switch your language preferences.
Am I making any sense?