Online language learning company Duolingo has finally added a Mandarin course to its offerings. With more than one billion speakers, Mandarin is the most popular language on the planet, but it's also on one of the hardest to learn, which is why the course will prove a little beefier than its European language counterparts. English speakers will learn the language's characters as well as the four tones of Mandarin, with lessons structured by themes such as greetings, food, health and sports.