Kotlin is just yet another guest language on the JVM, written in a mix of Java, C++, no Kotlin code around, neither today nor tomorrow.

Google is pushing Kotlin as hard as they can on Android, while leaving Android Java dialect stagnate on a pseudo Java 8 compatibility, they have a political agenda to play here.

Spring just goes after any shiny thingie that might bring users into their domain, Groovy, Clojure, Scala, and now Kotlin.