Indeed, it is the de facto language. In 2020, if you learn it, you’ll be able to build not just modern web applications but also any applications for any device and platform.
Getting yourself acquainted with syntactical stuff should not take you more than a week working 2 to 4 hours daily. Then you would have to go through the advanced concepts like the Closures, Web APIs, Event Loop, Prototype chain etc. This is gonna take some time. From one month to a couple of months.
Should I learn Kotlin or Java 2021?
Kotlin is the preferred language for Android development in 2021. Both Java and Kotlin can be used to build performant, useful applications, but Google’s libraries, tooling, documentation, and learning resources continue to embrace a Kotlin-first approach; making it the better language for Android today.