As with the chinese arguement, you have a few options:
Cosplay.com 日本語版 (Cosplay.com Japanese version) - this makes it sound like an application
Cosplay.com, 日本語のサイト (Cosplay.com, Japanese language site) - a little long-winded.
Cosplay.com 日本語 (Cosplay.com Japanese Language) actually sounds the BEST, IMO.
Since it's a title, it's kind of silly to use "no" between the Cosplay.com and the Japanese.
Again, all a matter of style.
Good luck deciding how you want it!