Even though our company has switched to 100% Kotlin, our applicants are not required to know Kotlin at all. It is sufficient if they understand the concepts, and have good experience in some language. Also they need to show some enthusiasm about wanting to work with Kotlin.
They can learn Kotlin while working… The same way we did when we switched to Kotlin some years ago.
Of course we ask them programming questions, but they can chose the programming language themselves. I would find it quite ridiculous to ask them hard to spot Kotlin puzzler traps.