Huh, Kotlin had almost completely slipped my radar but seems interesting from a quick glance now. I had always pigeonholed it as an "Android" language. Does it have general use?
Keep in mind kotlin is 100% compatible with java/jvm, so anything Java can do it runs on, kotlin does as well. Kotlin did get it's momentum from Android, but it's matured quite a bit. IMO it's a excellent language.
It has a pretty big ecosystem for all sorts of stuff, but on top of that it has native Java interop, giving you access to a huge selection of libraries.
There's also Kotlin Native, which compiles to native code using LLVM and ways to compile it to JS.
Basically, it runs pretty much anywhere.
According to dev surveys, usage is pretty evenly split between mobile and backend, but you could even do desktop or web apps with it if you wanted.