Well, really it's best for you to just work on whatever you find interesting. There's always more work to be done on Gnome (the default red hat 9 desktop), which is written in C. Check it out at developer.gnome.org
Python => useful to know, but currently isn't used much for desktop apps.
Java => not used much outside of enterprise servers
C/C++ => used for desktop apps
Perl => server stuff/scripting
LISP => not really used outside of emacs
C is a good language to know. Just find a project, and get stuck in.