Kotlin is great, but everytime I use something like that I get the feeling that my code is very inportable.Kotlin compiles to bytecode that HotSpot can understand, that lands you Windows, OSX and many nix flavours (Linux, BSD, Solaris, etc). As if that wasn't enough (it is), it also produces bytecode that ART/Dalvik can understand. That lands you a couple million devices more. If that isnt enough for you (for some weird reason, at this point you're just being unrealistic), you can always hook up libGDX, which is prepared not to only target all of that, but it also has a web backend (through GWT I believe), and an iOS backend (through Intel's Multi-OS Engine).
You can always bring up the "muh performancez!" argument against Java, but portability? That's silly. You're silly.
Disclaimer: Targeting all of those platforms is a **fucking ton** of work, regardless of the language you choose. So my position is that worrying about targeting 19 platforms without even having anything to run on them yet is also silly.