It looks to me like Kotlin is enough like enhanced Java that it's pretty painless to extend an existing Java codebase with it. IntelliJ even has automatic trans

It looks to me like Kotlin is enough like enhanced Java that it's pretty painless to extend an existing Java codebase with it. IntelliJ even has automatic translation of Java source files to Kotlin, though I'm sure there are edge cases where that doesn't go well. The benefit is not having to write Java anymore. If you don't mind writing Java then I think you might benefit from exposure to a wider variety of languages. As for getting confused between languages, most of us work in heterogeneous environments these days, even if it's just something like using Python server-side and Javascript client-side. I don't find confusion over the languages to be a significant difficulty there and I don't think it would be with a mixed Java/Kotlin project.