It should be possible to write the code in other languages then java
For example one should be able (it is not resonable but still should be possible) to write "main" sources using groovy and "test" sources using jython.
then once somebody types "maven jar" groovy compiler should first be invoked and used for compiling groovy classes then unit test runner which undestand jython should be invoked.
This probably also open the point if we should have something like <groovySourceDirectory> or <jythonUnitTestSourceDirectory>.
But probably we should replace:
with default language = java