Ive been very intrigued by Kotlin. As a language it's nicer than either Ruby or Python. Don't be turned away by the fact that it runs on JVM, there will be a native, LLVM based backend. The killer feature is that you can transpile Kotlin to JS so you can write your backend and front end in one language. You can also integrate with all the front end frameworks. Here's an official sample app based on React
https://github.com/JetBrains/create-react-kotlin-appAnother nice thing is that you have a good IDE for Kotlin. Kotlin is by JetBrains, the same people that built PyCharm and IntelliJ.