Vala is a language similar to C# that compiles down to pure C/GLib. It's quite popular in the GTK+/Glib community and it's substantially easier to learn if you don't know C and pretty awesome even if you do know C.  You can see an example of a Geany plugin using Vala in my Geany-MultiTerm repository[1].  The .vala files are code I wrote, and the .c/.h files are the compiled C files that you can compile with gcc or whatever you use.