To me the key functionally for any language learning app is giving you feedback on your pronunciation and general understanding. I’ve been using Duolingo to learn Mandarin and when I try to speak to anyone it’s difficult for them to understand me, because my pronunciation is all wrong. The app is just feeding info to me one way, and I can try my best to recreate what I’m hearing, but there’s no way to know if I’m messing it up. They do have a speaking feature but it doesn’t work very well, certainly not to the same level as speaking with a real person who is fluent in the language and having them correct you.