Updated 13 October 2024
DuoLingo is a language app available on Android and IOS as well as a web browser version. I have been using it every day for nearly two years to learn Ukrainian, usually on my phone (Android) and sometimes on my iPad (IOS) and once or twice on my PC. My experience, therefore, is based on just one language, so some of my comments are specific to that, whilst others are more generic.
