[_huayra_]

It's hard to learn a language for fun, so I'd pick something that fits your needs to build something (or even just your curiosity). Elixir and Prolog, although both cool, might not fit the bill because they really excel at one particular thing.

Golang is a popular answer, as you can start building stuff with it fairly quickly (especially compared to Rust). Java can also be useful if you haven't learned it and find a use case (although you will hear it bemoaned as the "New COBOL", there is still a lot of work done using it).