So Javelin is written in Java, not in Kotlin.
I don't mean to sound unfair
but isn't it that saying "a Java/Kotlin blah blah" just a kind of marketing trick ?
If I get Kotlin right every Java solution is also Kotlin compatible.
There's a difference between just being compatible and nice to work with. Javalin is being developed with Kotlin in mind. I'm a Java dev who wants to transition to writing Kotlin, but most companies are not very eager to make the switch. Javalin is intended to help ease this transition.