Kotlin is an statically typed language that shares many similarities with the likes of Scala and Swift. In a sense, it could be thought of the equivalent of Swift for the Java platform: a more modern, concise and safer language that runs in the JVM, fully interoperable with Java.
Recently it has seen a big spike in popularity thanks to Google officially adopting it for Android development, alongside Java and C++.
The interesting news is that Kotlin can be used with SFS2X on both client and server side painlessly.
Learn more about it in this article from our blog