I love Japanese as well. Norse is also one of my favorites, and my absolute favorite is Slavic. Although I refuse to write about Slavic mythology until I build up a follower base, but I will have both Slavic and Norse mythology series at one moment in future :D Celtic and Chinese seem interesting but those are the ones from which I have only scratched the surface.

I started with Greek but it soon dropped from my favorites as I started reading about the Norse mythology :)