Duolingo /ˈdjuːɵˌlɪŋɡoʊ/ is a free language-learning platform that includes a language-learning website and app along with a crowdsourced text translation platform and a language proficiency assessment center. Duolingo is ad-free and offers all its language courses gratis. The language-learning website and app offer over 40 different language courses across 23 languages. The app is available on iOS, Android and Windows 8 and 10 platforms with over 100 million registered users across the world.