The engine and everything else is C++.
"And for others that also don't know, a great example on how to do cool UIs with Java"
This is vague, and could mean either it's a "cool GUI app (end-to-end) in Java" or "cool GUI app (with just the GUI) in Java".
Think how one could say the same thing you said for a 100% Java app too, e.g. "jEdit/NetBeans/etc is a great example on how to do cool UIs with Java".
I added the explicit information that the Java part is constrained to the UI, and clarified what the backend is written in.
The engine and everything else is C++.