I can't think of any popular language that would take more than a few days to get acclimated to as an experienced developer, so that's not a very compelling argument.
It's always the (usually quite bad) tooling, learning about platform/SDK shittiness and pitfalls, and figuring out which parts of the open-source library ecosystem you want to engage with, that takes like 90+% of the time getting decent with a new language, in my experience. Getting comfortable with the language per se takes low tens of hours at most, as you wrote.
Kotlin (the base language) is really not that different from java. I went from 0 to standing up new backend services with limited friction. Coroutines and maybe frontends are a different story. Java doesn't yet have a coroutines equiv so that was a larger hurdle for me.
Most of the changes for me from 10/20+ hours to now we're more about identifying a style that works as effectively as I can. These types of behaviours are normal in all but the most idiomatic languages, so if anyone is doing java dev as their daily language, Kotlin felt very natural(though you really are limited to Intellij since the IDE does a ton of lifting to make your life easy).
Well, C and Scala are some counter examples that immediately come to mind.
Kotlin is probably more similar to Java than any other mainstream language. There’s almost no learning curve there, while going from Java to other “easy” languages like Python requires significantly more time to get used to.
Scala it depends how you want to use it. If you're going for full FP then sure it can take a little bit longer, but you can also just use it like Java+ if you really want...
I think there's a difference here between getting acclimated to scala (for new code, presumably), which is reasonably easy, and getting acclimated to a scala codebase that was already written by someone else.
You can do the first one basically the same way you'd do kotlin, the second one can get pretty hairy if someone decided to bring in a bunch of macro heavy DSLs and syntax extensions.
Ooof, I think most devs can modify an existing code base in a few days. To learn all the idiomatic styles, tradeoffs of the major libraries and different build systems take months, maybe years IMHO.
I can't think of many other languages that will compile into a Java codebase, and be interoprable in both directions, as well as Kotlin. It's a lot quicker to pick up than e.g. Scala IMHO.