Haha, I'm glad I saw this. I'm in the same boat where I'm learning Japanese and was toying with the idea of making an app to store resources and help others learn as well.
I've just downloaded the app to give it a try, and certainly I like where your going with it. The UI is simple and clear, it's nice to have something that isn't to 'in your face' with bright flashy colours etc. The phrases section is all laid out really well (Maybe audio playback on phrases at some stage?). A easy access grammar section is definitely worth having, will be good to see more content in that area.
Only thing I would say with the writing system section at this stage is that for reference purposes it might be better to have the tables of hiragana and katakana accessible via links directly from the main menu for easier / quicker access. Also I just quickly tested this on my phone (HTC One M7), and when holding it portrait the tables don't scale to the screen width, so you have to scroll across to see the full table of characters. Again though I know this is still early stages, so you've probably already got all this planned in an update lol
Anyway, great to see you are learning Japanese as well and that you are making something to help others learn as well!
I look forward to seeing how it progresses as you update it.