As mentionned in post #16 here: http://www.vbenterprisetranslator.co...-french-2.html
Maybe, another option to make sure users have set a default language, could there be an option that would detect if a user has done it when he logs in on the forum? If default language is detected having been set, user would continue the log in process as usual but if it's detecting it as he did not, there would be a message requesting the user to go in their profile (bringing him there automatically) and asking him to select one default language before he can go anywhere else on the forum.
This would make sure all messages would be translated (including BBCodes) on the forum because all users would have selected their default language.
This would really be a great addon.