Duolingo’s Luis Von Ahn on How the Language App Added Africa to the Mix

Even to Luis Von Ahn, there are a few things about Duolingo, the mobile-phone based language learning app that he co-developed five years ago, that don’t make a lot of obvious sense. Like the fact that bilingual people tend to study languages they already speak fluently. Or that the easiest language to learn — Esperanto — is one that no one speaks at home (German, by the way, is one of the hardest to learn).