Study quest is a spaced repetition language learning app created by the wonderful Futuretro studios.
I had the privilege of handling the English to Japanese translation and even designed several of the boss battles you'll encounter in the game's safari mode.
The app is packed with features that let you study the way you want to. But, perhaps even more importantly, it's engaging enough to keep you using it long-term.
You can download the app absolutely free on Android and iOS. Find out more about it HERE