Localazy

It really depends on your needs but with a decent knowledge of java, you can learn Kotlin quickly and it's possible to use it both for the backend and frontend.

At Localazy, Kotlin is our primary backend language but we also used it for our CLI tool and with a single codebase we are able to produce native binaries for Windows, Linux, macOS as well as Node and JVM.

Also, I used Kotlin for smaller hobby projects several times and easily wrote the backend part and some dynamic frontend components with it.