Please open any Pull Requests into the develop branch or ideally into a new branch for your changes. PRs that go into master won't be ignored, but I have to checkout and merge manually, which makes your PR show as being closed.
Do not write any Kotlin-only APIs; all API components should be written in Java, Kotlin extensions should not have functionality that isn't available in java. The same applies the other way round, do not write any backend code in Java, it should be Kotlin-exclusive.
If you have any questions about contributing, feel free to ask in the Discord!