Haskell → dunno, but it’s pretty cool
Erlang → dunno again, maybe some low-level stuff for Elixir?
F# → business apps for MS when you get sick from C#
Crystal → web, I guess?
Ruby is good for web, but it’s also useful when metaprogramming tricks work for you in your particular domain. Same with Python (it’s also insanely good for web!).
(and that being said, you can also use Erlang libraries in Elixir, so there’s a path for incremental adoption)
Or at least that was the case some year ago. Haven't checked lately. I know there are ongoing efforts towards convergence.
Neither option allows you to use Phoenix without writing Elixir code, though. But since you’re integrating an Elixir lib, writing at least some glue code doesn’t seem like a big deal?
(I’ve got to say that I’m not proficient at all neither in Erlang nor in Elixir – I’ve played around with both, but nothing serious.)