We're using Kotlin in our backend and pretty happy about it. It has support for co-routines and great syntax, it's lightweight compared to Scala and IntelliJ plays great with Kotlin (no surprise as both of them are from JetBrains).
I won't even mention the learning curve, we started learning Kotlin using IntelliJ's support for converting Java files to Kotlin initially and then it took only one week to learn about the details thanks to its documentation.
I won't even mention the learning curve, we started learning Kotlin using IntelliJ's support for converting Java files to Kotlin initially and then it took only one week to learn about the details thanks to its documentation.