If you want to learn Spanish, start to learning Spanish, not Latin. Same for programming languages. There are two things from Java that are good to know:
The first thing are APIs. But you need only a general overview about what exists. Even long time Java programmers don't know all the details. And finding the right API or lib for a problem is usually easy, as Java is so common, and even with weak Google Fu you shouldn't have any problems.
The second thing you need to know are some basic principles and limitations of Java and the JVM (including how to build and run), that help you to understand some of Scala's problems and design decisions. One typical example would be "type erasure": If you don't understand this limitation of Java's generics, you'll run in problems when using generics in Scala.
As you can see, the things you really need to know is limited. Everything else can be picked up on the way.
