There are many apps that have utilized formal methods in an attempt to teach languages as optimally as possible. But Duolingo is still the leader in language learning. Why? Language learning is an emotional process. Every word you can bring to mind likely has some specific memories tied to them, from another time and place. So even though Duolingo is far from optimal in terms of how and when to present new items to learn, it is close to optimal in vibes, and apparently in the market of language learning this is what consumers prioritize over all else. I believe it is for good reason. Whoever displaces Duolingo will do so not because they teach more efficiently, but because they improve on embedding particular emotions and sentiments into the lessons.