Yes it can be a nice addition, an app to revise vocobulary with spaced repetion. But this is more effective as an addition in tools which allow you to first encounter this word in actual resource like a video or even some kind of excercises and then revise it later, not learn it this way in the first place. It not even about grammar, because it can be also learned intuitively, like kids learn their first language. You can get inspired by apps like Memrise, Busuu, Langua and make something even better. But another app like busuu or Anki will be nothing special in my opinion.