1. In Java lots of libraries and plenty of examples.
And none of the library to be rewritten in Java. The advantage of Kotlin in the compatibility with Java all these libraries can be used from Kotlin. Moreover, Kotlin would not take off without the ability to use Java libraries.
That is, if not to write, to read Java you should be able to.
2. You overestimate the knowledge of the programming language.
For the programmer it is natural to know and use several of them.
Well, for example, the most that neither is ordinary web development and already:
PHP, JS, SQL, HTML, CSS - 3 first is the programming languages.
And it is at least.