The startup Duolingo has found a way to make free foreign language learning profitable.

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The company, which counts Ashton Kutcher and four-hour obsessive Tim Ferriss among its backers, is doing it, in part, by enlisting new language learners to translate the web.

It’s an approach that investors clearly like: The Pittsburgh-based startup today announced a new $15 million funding round led by NEA and Union Square Ventures. Duolingo launched in June with $3.3 million in backing from actor Kutcher’s A-Grade Investments, Union Square Ventures, and Ferriss.