Maybe it's because I know both languages but is that really a big issue for people? The interop is great, and kotlin is very readable, so the cost of context switching between the two is miniscule.

Some people have an extreme aversion to learning new things though. I feel that holding yourself to the standards and limits of your lowest performers isn't a great thing.

Sounds like you're making progress, your devs are slowly learning a better language that will let them work faster and will soon be able to help port the rest of the codebase and then you can really accelerate when no one needs to touch or know Java.

Doesn't Kotlin has interoperability with Java? I didn't used it much yet but I'm about to in a few months. Is it that difficult to just refactor things to Kotlin when you need to change something in the project? I'm asking because I just can't work with verbose languages and would prefer Kotlin to Java everyday.