In 2009, Severin Hacker and Luis Von Ahn were holed up in the computer science department at Carnegie Mellon University, turning over a seemingly impossible challenge: how to translate all one trillion pages on the Web, which are mainly in English, for people who speak other languages.

Neither Hacker, a native of Switzerland who was then a PhD student, nor Von Ahn, who was raised in Guatemala and served as Hacker’s advisor, was very impressed with the options. Feeding Web pages into Google Translate usually generated gobbledygook, while trying to hire enough translators was impossible. “Translating is a task that people don’t want to do,” Hacker says. “It is work.”

So Hacker made a game out of it. Known as Duolingo, it teaches foreign languages to anyone with a smartphone or an Internet connection, for free. Unlike most language classes, with their reliance on rote memorization, Duolingo offers constant interaction. You respond to multiple-choice questions and complete sentences by typing in answers, and you practice phrases by speaking into the microphone. If you answer incorrectly, the app shows you where you went wrong; if you make too many mistakes in a section, you’ll have to repeat it. Each course takes around 35 hours to complete and promises intermediate-level proficiency.

But the real genius of Duolingo is the way it solves the problem that first stumped Hacker and Von Ahn. When you reach the highest levels of a course and translate sentences into the language you’re learning, Duolingo compiles your work with that of other students. That aggregated work tends to produce accurate translations, and media companies such as Buzzfeed and CNN are paying Duolingo—now spun out as a company, with Hacker as CTO—for foreign translations of their English Web pages.

Duolingo offers courses in 30 languages and counts 30 million users. Hacker himself recently used it to learn Spanish in order to travel to Guatemala for Von Ahn’s wedding. Relying just on the app’s lessons, he navigated the airport, hotel, and restaurants, read the newspaper, and got a haircut. “Ten minutes of Duolingo,” he says, “is worth, like, an hour of class.” 

—Patrick Doyle

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