Duolingo gives first class support to Spanish, French and German. Pretty good for Italian. The Welsh course has good voice synthesis. By comparison, the Scot's Gaelic assembles spoken sentences like a mad bomber using discrete words spoken by males, females, old and young just stitched into sentences. The key difference is some languages exist because dedicated volunteers put effort into creating them. Others get the "full corporate support" including voice synthesis, voice recognition, reading exercises, listening exercises.
I started into Mandarin Chinese on Duolingo, but it was a pretty steep learning curve. The Hello Chinese app on Android did a better job of helping me learn the tones in Mandarin. I made good enough progress in Japanese that I was able to read the hiragana/katakana wall graffiti at my local Japanese sushi shop.
Duolingo implementations vary. The Apple app gets the best toys first. Android implementations are good, but run far behind the Apple development. There is a web version that works for desktop computers with just a browser. The web version has other features including forums with other learners and instructors. Sometimes there are group audio chats.
Duolingo discontinued the forums, although they are still archived. Which is really disappointing because that was actually one of the best things about it. Generally if you have a question, if you went through the forum, you would find your answer.