Duolingo is probably the first app new language learners encounter, but Memrise is not far behind (https://www.memrise.com/). And, for some folks, it may be a better choice, mostly because it offers a huge variety of languages…some very obscure to Western ears, like Telegu (from southern India). I have Indonesian (supposedly the easiest language of all to learn) as one of my choices, as well as Spanish.
The interface is similar to that of Duolingo, but significantly different at the same time. Hard to explain, but you know you are definitely in a different tool. For one thing, it’s less cartoonish than Duolingo, which will appeal to some who are put off by Duolingo’s approach. At the same time, Memrise uses spaced repetition and aims to help you learn five new words per day in your target language. There is an icon of a tree branch on the right-hand side of the screen, which gets a new leaf every time you successfully learn a new word. Additionally, there is an audio icon, which allows you to hear the word, or phrase, as many times as you desire – and, you are also expected to be able to read the word before you are considered to have learned it.
I use Memrise in conjunction with Duolingo because it exposes me to different words. As I indicated before, there will be those who prefer one over the other. But, in my mind they are basically the same app, with minimal interface changes. I suppose some will consider Memrise to have a more “adult” approach. Personally, that is not particularly important to me.
Memrise is free, and it is available on iOS, Android, and on the web
(https://www.memrise.com/). I could not find it in the Windows store.