A powerful trend in Duolingo's business could accelerate thanks to artificial intelligence.Duolingo (DUOL -1.62%) is the world's biggest digital language education platform. It takes a mobile-first approach, placing gamified and interactive lessons at the fingertips of millions of users worldwide.The company operates using a freemium business model -- anyone can access its basic tools for free, but those who want to accelerate their learning can pay for a monthly subscription and unlock additional features. ...