Learning a new language is tedious and demands a lot of practice. Luis von Ahn doesn’t want all that effort to be wasted. In fact, it might be a gold mine. Von Ahn, a computer science professor at Carnegie Mellon University, is the co-creator of Duolingo, a free language-learning site that turns students into an online workforce. His software uses their answers to simple exercises in a translation service that he expects to charge money for.