While there isn't a Japanese course on Duolingo yet, I wanted to share with you all the Memrise course I made for Japanese! It is based on the English for Japanese speakers tree. I made it to go with Demon-Kiyomi's guide to Japanese here. So, if you've wanted to learn Japanese,
Here's the Memrise course: http://www.memrise.com/course/1073183/japanese/
And here's Demon-Kiyomi's guide: https://www.duolingo.com/comment/7963862
I would reccomend using both resources together, because Demon-Kiyomi is better at explaining grammar than any Memrise course, and the Memrise course will help you go actually learn all the words!
Edit: I just remembered how my course doesn't teach hiragana or katakana! Here's a Memrise course for that: