Another subject-area-specific option: Elixir. I do like every option you've presented as well; Elixir just fills another niche.

I was considering mentioning erlang, but you probably don't want to write erlang. I've never touched elixir, but I would likely try it if I hit a problem that I would use erlang for in the olde days.

how you define mainstream?

by number of jobs, scala is way ahead, also python, javascript, ruby, java.

Even here, when jobs are listed, numbers of go jobs are quite low

Might be bias from my anecdotal experience, but whenever I speak with people working at large corpos, go comes up more often than python or ruby.

Whenever i see a bunch of code written in a language different fron whatever the rest of tge team are using, that language is often: go :)