I've just tried it (japanese). May I suggest adding (optional) furigana for japanese? The problem with kanji is that you can't read a word you don't already know.

Edit: it seems like there's quite a bit of colloquial japanese, which is good (at least for me) . How did you build your corpus?

Edit(2): there are lots of bad single word translations (the underlined ones). They either change spontaneously between two fingerpresses, or are just plain wrong.

Also, I don't mean to be harsh, but I'm not sure what your app brings that duolingo and kitsun don't already do?