> Not as an end by itself, no; I want to learn more languages out of sheer intellectual curiosity, but most of all because I like building things, requirements vary by project, and I believe that it's good to have different tools to tackle them.
> I work primarily in JavaScript and Python, but academically I've had rigorous exposure to C, Java and OCaml, and a few other languages to a lesser extent. I appreciate the benefits of dynamic languages but I sometimes miss the confidence and expressiveness of strong types.
If you've actually made serious use of OCaml then I wouldn't bother with any of the languages on your list - they're very much the same paradigms as OCaml (similar to how I wouldn't advise someone to learn both Ruby and Python - it's not that there aren't differences between them, but those differences aren't mind-expanding if you see what I mean). If you're talking about "a multi-paradigm language with strong typing to my toolbelt" then isn't OCaml already that? So I'd say Elixir is a better bet from that perspective - I'm no fan of the language, but it does something that none of the other languages you've talked about does. (Well, there is an actor implementation for Scala, but IME it's in an awkward tension with the rest of the language - kind of like the OCaml object system in that respect).
> Not necessarily growing as in the left side of the hype cycle, but in that it will likely continue having a vibrant ecosystem in 5-10 years time.
Scala is not growing fast, but it's mature and stable in a way that Kotlin isn't yet; I genuinely have more confidence in Scala being usable in 5-10 years' time, though that may be my biases showing. I'd say the same thing but to a lesser extent with F# - the whole .net ecosystem is kind of a parallel world from the big contiguous dev community, and the move to .net core is a big disruption at a time when F# is always going to be a lower priority than C#. (Of course the JVM looked equally rocky for a while, but it feels like the community has coalesced smoothly around AdoptOpenJDK now).