Esperanto. A made up language that is really easy to learn and spoken to some degree by about 2 million people all over the world. I got into it when I heard that if you speak it you can stay with Esperanto speakers that just want to practice with strangers, for free. I traveled all over the world for free and met so many awesome people.
When I try to get anyone to learn it, they just won't. They hear about that criticism of the language or another, or plain get bored. You can just start the Esperanto course on Duolingo for free, but nobody I know goes through, despite the benefits.