Duolingo.
You already know, the go-to for language learning. Duolingo has a large selection of different languages to learn from and it's all completely free! They're constantly adding new languages and recently began offering Hawaiian and Navajo. I started learning Norwegian with it and keep practicing my French. I will say that for Asian languages it's a little iffy. I wouldn't recommend it for Japanese in particular because it introduces kanji very early on without giving any background about kanji. But for most languages, I would say this app is top-notch.
Tinycards.
Tinycards is a flashcard app and you have the option to download card decks that people have already created. This app was my savior when I backpacked Asia. I would download a deck with basic expressions (with that free airport wifi) and then I could practice the vocab on the flight to my next destination. The app was actually created by the same developers at Duolingo! So unlike Duolingo, this is actually a little better if you're just trying to pick up some vocab from an Asian language. I successfully learned how to say "please," "thank you," and "hello" in Thai, Vietnamese and Filipino!
Drops.
I haven't really heard many people talk about this app but that doesn't mean you should be sleeping on it! This app is set up like a game and includes pictures for the vocab too. It's basically one big matching game, and also involves spelling. The app is set up so that you have just a minute of language practice a day but you can do as much as you want within that minute. The downside is that you only have one minute a day. If you want more practice you have to pay within the app. I think the minute is pretty good especially if you do it every day, but I wish that it allowed a little more time.
Quizlet.
You probably already know about Quizlet. You probably already use it for one of your classes in school but just in case, Quizlet is basically the same as Tinycards. The difference being that Quizlet has a much bigger audience than Tinycards so there are likely to be many more contributors. So if you're taking a language class or studying for a proficiency test this may help you out more than Tinycards because you can learn decks specific to terms that may be on a test. You can also use this to learn more about history or just things about the culture in general because it's not limited to language learning.
Memrise.
I'll be honest, I have a love-hate relationship with this app. I currently don't have it on my phone but I remember that I did like it. This is another paid option but I personally really enjoyed it for the time that I had it. It was pretty good for learning Japanese and has a whole host of languages you can pick from. I remember it being pretty similar to Duolingo but I think if you're thinking of investing in a language learning app I would say this is a pretty good option.
Wanikani.
Wanikani is for learning kanji. I used it for Japanese and I'm not sure if you can use it for Chinese because of the different meanings but this is a pretty tried and true method for learning kanji. It takes a while, you can't really progress at your own speed because the app will only let you progress at their own speed (it's part of their learning method) but it does make sure that you really know the lessons before they let you move on. I have fallen off the wagon with my kanji practice but I'm looking forward to getting back on track using this app.
Lingodeer.
Lingodeer is yet another app I tried for learning Japanese. It also offers other Asian languages like Korean or Chinese. What I liked about this app was that it included recorded conversations of an actual native speaker so that you could hear the pace of the words and where they put emphasis. To progress in this app you also have to pay, which I am not so much a fan of, but they do offer a lot and have a different approach than a lot of other language learning apps.
TLDR: If you're looking for a good free app to learn a language Duolingo can't really be beaten; for vocab, I would recommend Tinycards and for Kanji, I would recommend WaniKani. For a paid app, I would recommend Memrise or Lingodeer for an Asian language.
