As the most spoken language in the world, I feel like it would be really cool to have a Duolingo course on this. Is one in the works?
Even though I think a language such as Chinese would be hard to fit in Duolingo's system, I'd be really glad to see it added. But while it isn't added, there are a lot of other sources to get started with the language:
HelloChinese: you learn to write the characters and learn some everyday phrases and words. It is very didatic and simple to use.
HelloTalk: I use it a lot. You talk to Chinese native speakers that want to learn English - or whatever your native language is. Very good to get the grasp of the language used by people on the internet.
Anki: a flashcard-based app although it is a bit "crude", it is a great source for vocab learning. It uses a Spaced Repetition System (SRS) that is way better than Duolingo's. For disciplined learners, it is probably the best tool to learn fast and remember the Hanzi. You can make your own flashcards very easily (my own deck already has 1500+ cards).
Memrise: its SRS is not very good compared to Anki (still better than Duolingo's), but its system of mems (funny, interesting or unexpected mental links to learn every single word) is worth a try. I've stopped using it to learn Chinese, but I still use it to learn other languages. If used together with Duolingo to learn European languages, it will help you tremendously, as it's been doing lately with my French.
Yeah, as a native Mandarin speaker, I really wish that Chinese can be added in the course list so that more people can learn it. Yet it's really hard to do so since I tried the "English for Chinese speakers" course and figured that languages that can switch the order of words without causing confusion (such as Chinese) are quite hard to fit into the system. When I was trying to "learn" that course (I know both languages quite well so I was just curious), I found out that it's quite hard for me to get the right answer. Not because I didn't know what the question was asking, but because in Mandarin, a word can have several meanings. Also, sometimes if you switch the order of words in a Mandarin sentence, it also makes sense. However the system didn't add all the possible answers, especially the colloquial ones. So if "Mandarin for English speakers" would be added, I hope this problem can be fixed somehow.
Many kids are learning this language in school and need support especially with their coursework, there are very few ways to effectively learn Chinese and it is even more difficult if students do not have a family member that speaks the language. A course in Chinese would help eliminate this problem and provide feasible help to students (and non-students) who want to learn another language.