General principle:
Everybody can learn a new language easily and for free on Duolingo
Technical overview-API version:
Duolingo already uses an API. Since the app focuses on learning a language, it is directly connected to a dictionnary through an API so the words can be checked
maybe another way of using API could be by linking Duolingo's users to other websites, or maybe videos which would focus especially on certain lessons, or a precise vocabulary. The ones who are motivate to learn more or the have more precise lessons concerning grammar for example could do it, just by clicking on a link!
Added value thanks to APIs:
APIs are central here: they allow to have a quicker feedback for every user after each exercise. It allows Duolingo to offer a true personnalized experience which helps motivate everybody!