there's a very good web app (it works on mobile browser as well, but no native app) on japanese. please someone make a mandarin version that works like this http://nihongomaster.com/

For Kanji, certainly. It probably won't suit Hiragana and Katakana.

Yes, for Kanji. These hieroglyphs is what most people are having trouble learning in Japanese and Chinese, as far as I know. Grammar is hard but it's not such a barrier.