That would be.. Java :D While I’m quite a big Java fan, for the other parts some other JVM language can easily fit the bill and you even get a choice. Scala, Kotlin, or maybe even Clojure if you don’t mind dynamic types.
The others look like really really nice languages and I'd love to use Scala for a solo project.
However I guess I need something that's a) truly general purpose b) not going to scare away collaborators or potential hires c) going to be around for a long time d) not going to require evangilization at, say, a start-up.
Kotlin... I guess I'm wary of it being the new Coffescript - i.e. that it spurs Java into improving to such a degree that it removes the need for Kotlin in the first place.
And yes, having just spent a long time transitioning a project from JS to TS, I'm not sure I want to start anything serious without at least half-decent static typing - a major reason I'm looking for an alternative to Python, which is just not quite there.
I think that's unlikely. The key difference being that Coffeescript was always dependent on JavaScript because it compiled to JavaScript! Whereas Kotlin doesn't compile to Java, it compiles to JVM bytecode the same as Java does.