As someone who's been in that field for the last, uhhh, 8 or 9 years, I'd say feel free to go with Kotlin. You will understand java the moment you see it later on, meanwhile Kotlin is easier to write, understand and think it. It will get you where you want to be faster than Java. All modern examples will be written mostly in Kotlin, so you won't have trouble adjusting. But in the end, just pick one and go with it, being paralysed by analysis won't help you at all, and after you're fluent in one the other will be familiar too.