They're still either Japanese or Korean because that's the two languages they sing in, bar the couple of English songs
The Japanese thing just means the songs in Japanese, which means it's on one of their Japanese albums They have two Japanese albums, Girls & Peace being their second one.
Was it Boomerang you linked before with a leaked Korean version?
I should probably go to bed, alarm goes off in little over 5 hours.
Do they speak Japanese fluently or is it more like the Korean "I can sing in English, but not that good" kind of Japanese
They can speak basic Japanese, they can actually speak English more fluently than Japanese. Well, not all of them but some (Jessica, Tiffany and Taeyeon at least) the other members might be better at Japanese. They know enough to sing in it and have conversations but not quite fluent I think.