I was listening to an interview Sunday of Luis von Ahn, CEO and founder of Duolingo, the language-learning company. He said that when asked by his investors about charging people for the service, he would not waver. "We're never going to charge people."   But he knew that he would have to figure out ways to monetize it—the popular app is now valued at $1.5 billion—he told NPR's Guy Raz on How I Built This.   "We were very precious about this," von Ahn, who is from Guatemala, said. "We cared a lot about the user experience for Duolingo. And first of all, we thought, 'We're never going to put ads in here.' But we had a year of turmoil, and we needed to figure out how we were going to make this sustainable. At some point, enough [employees] got convinced, 'Okay we'll put an ad at the end of every lesson.' Fortunately, that gave us quite a bit of money [$10 million annually—they use Goog ...