However, why support both Java and Kotlin, it seems that would create more work with no real need ? Admittedly, I've been burned in the past with Play! which said they would support both Java and Scala but using the Java version I always felt a lot behind in feature support.
Play was two different projects tho? There is only one Javalin source.
In Javalin, although some parts are written in Kotlin and some parts are written in Java, there is 0 duplication. If one part is written in Kotlin, both Java and Kotlin developers use that part. If a part is written in Java, both Java and Kotlin developers use that part.
Also, Kotlin and Java are meant to be well interoperable, so the Java support sort of comes by default. So it won't be the same as Scala + Java. I've been burnt by Apache Spark which has an awful Java API compared to Scala (and Python).
If someone has a better answer, I'd love the education :)