Duolingo is quite terrible at teaching a language. It gives absolutely no context of grammar or conjugation besides what it beats into you through repetition. Don’t get me started on their “pronunciation recognition”.
It is pretty much only valuable as flashcardish games for vocabulary building.
Babel is expensive, but much better.
Actually textbooks are even better if you are dedicated to learn, but of course the best is a combination of that and a language partner and native person to talk with.