One forum login to rule them all (languages)
What I can think of:
- different database
- different cookies
- different sessions
It is possible to create a single cookie or single session regardless of the sub-domain but it is a PITA to implement, and usually causes more problems down the road than what advantages it can provide.
What they could have done was provide a single forum for all, and just let users set an option on which language forums they want to see. But it seems that it is not a feature of the forum software they are using, which is fangamer forums (though they can add it themselves).