Truffle Debugging Demo: Debugging multi-language applications [video]
youtube.com
youtube.com
And Truffle can do some amazing things with cross-language that aren't related to debugging, see http://chrisseaton.com/rubytruffle/cext/
I think we will eventually end up with much of Java implemented in Java, and that will make for fantastic dynamic language support, but it's going to be a long road.