I think Javascript is generally a safe bet for the next few years because the Frontend is firmly embroiled in Javascript as well.
Elixir seems poised to keep growing on the backend, so it's probably not a bad one to learn either.
My real advice to someone starting out would be to learn a language but focus on learning general programming and computing concepts through it. The better you are at programming and computing, the more easily you can pick up new languages and switch between them seamlessly.
Also, as you learn new languages, focus on learning the right way to do things in that language. It's extremely helpful to be able to separate programming concepts from language constructs.