I think you can't learn to be fluent in a language with just the app, since it wouldn't correct you when you say something wrong for example, and it probably wouldn't teach you all the grammar rules. Imo, I think it's good most of the time, but then sometimes you need to practice with someone who's a native speaker of the language so they can guide you, and hey, they might even tell you if you're saying something wrong, which is nice :D.