This looks cool and I will try it out.

I'm curious: have you tried out Anki and Memrise? Both of these have free Android apps, sync progress to a central server and, more importantly, have pre-made and/or user-submitted 'decks' for common languages and texts.

Did you build this because you wanted something simpler, or were they missing an essential feature?

EDIT: I see from the last of the screenshots shown in Google Play, that you have a neat feature: show me a notification about a word that I'm ready to practice (or about to forget). Anki and Memrise don't do this, although Memrise will occasionally remind you to come back.