Duolingo taps into crowdsourcing to teach a language and translate the Web... His free Duolingo app and website invite users to learn one of six languages for free. The gamelike interface is ad-free and encourages users to compete with friends. But here's the thing: After about 45 minutes of instruction, the program begins asking users to translate random sentences into their new language. Von Ahn plans to sell the sentences to sites that want their material translated on the Web. He expects that half of the service to be up and running later this year.