Kinda hard to explain but it is a all or nothing at all thing.
Meaning you can pick the default language for the board. Via your profile or control panel.
But it will not translate the existing posts to another language.
Most all will translate the generic stuff like navigation bars,etc
That are in the language file.
So it could be done but what ever the default language is will be what is used for registration,navigation,pm's etc.
The best example would be install another language pack change your profile preferences and see the changes.
