With multiple languages working on the same JVM now, it is possible (and easy) to use different languages for your production and test code.
Why not to take this opportunity when it can be easier and faster to write test code. This can be also a good way to learn a new language. You can keep your production code using the language you are the most comfortable with and write your tests in Scala or Groovy.
This post describes how to configure Maven project to use Java as a production code language and Scala with a ScalaTest framework for unit tests.