The only way to get joint compilation across JVM languages would be if every language supported an explicit stubbing mechanism. This is not the case, so joint compilation of Groovy and Kotlin is not possible
- you can jointly compile Groovy and Java
- you can jointly compile Kotlin and Java
- you can serially compile any combination (but that means dependencies can only go in one direction, e.g. Java < Groovy < Kotlin)
If you want an honest opinion: Don’t create a tower of babel. Stick to one alternative general-purpose language per project.