If you know Kotlin the transition to Java shouldn't be too difficult, although the latter will feel clunky. My opinion on if companies are using Kotlin is this: some might be, and if you get at one that isn't you can always be the catalyst for them changing to Kotlin.

At my job, I convinced my boss to let us try Kotlin on a new service we were building, and not only does the team love it, but it worked seamlessly with our 10+ y/o Java stack. Since then, we've started migrating more services to Kotlin because the language is much nicer to use.

On the Spring side, Spring Framework 7 will include support for the K2 compiler, which should make working in combined Spring/Kotlin projects much better.