It's not Japanese-only. It was localized in English, and is currently available on both iOS and Android.
Essentially, it's a puzzle game where you collect/level up monsters. These monsters have different skills and abilities that are, often-times, suited for a specific purpose. It can become addicting to go after that elusive team you're trying to build, especially when a few of them are REM [Rare Egg Machine] pulls. REM pulls are eggs/monsters that cost five Magic Stones each but, as I said, the game is pretty generous at giving you out free ones, if you're patient. Wait to pull during a Godfest, though, as the odds of getting the specific monsters present in it are increased to 3x the normal rate.
It's highly addicting.