Please correct my bad assumptions, and make sure I have analyzed this problem correctly. I don't want to re-start once I start.
Perhaps when I am done writing it in Python, "Shed-Skin" would be mature enough to use. Risky!
Personally I like Python with wx.Python for GUI stuff. Runs on Windows, MAC, and Linux just fine.
Within the Java orbit, there are also Scala, Clojure, Groovy, and Jython. Scala gives you all the power of Java with a much more powerful syntax. Clojure relaxes the typing constraints without too much sacrifice in performance. Groovy is a scripting language designed to appeal to Java programmers. Jython is a Python on top of the JVM.