Translate the source code of Xfce.
How to start writing or translate the application manuals on this website.
How to contribute code and becoming a developer.
Additionally, Xfce uses the GTK+/Glib family of libraries which can be found at the following:
Tutorials for getting started with GTK+ are available at:
If you're looking at writing a panel plugin, you may want to look at the xfce4-sample-plugin for an easy start.