Kotlin does not use many Java libraries like it originally did. The only one I have used recently is the Random class to provide random numbers. Every thing else has a pure Kotlin class.
The course here is incomplete so finishing either Python or Java first will help your understanding Kotlin and the things missing. Since Python has operator overloading like Kotlin and Java does not, I suggest finishing Python first. Many Kotlin class libraries are similar to Pythons.
I really think it depends on what you want with the languages, though both languages work for Android development but Kotlin preferred by Google for now. Plus I need to say that Java has more libraries.