I don't know of any forums like this that are completely in Japanese, but in this FCDev section you can ask questions in Japanese. Let's see, uh, Koko ni nihongo de kiku koto ga dekimasu. I think I said what I meant to say
. But not very many people speak Japanese here. I'm studying, and have to google over half of the kanji I see. Quietust, who used to be one of the most active users here, and a moderator, was quite fluent in Japanese and often got on my case for attempting to speak it when I couldn't
. But he's been gone for a few years now... I guess if you wanted to ask stuff in Japanese, you could and a few of the users here (Banshaku for example, who I believe lives in Japan) would be able to answer your questions. I'd try and understand what was written and formulate a decent response, and it would be good exercise for me, but then you might as well be speaking English!