Java

After all the bad press it is getting right now? Does anybody still have it installed

Probably a better bet in C, as that was the common programming language for GNOME2. The source code for the MATE applets is here:
https://github.com/mate-desktop/mate-applets (all in C). So you could pick one that looks a bit like what you want to do, and perhaps copy & paste to get it to where you want it to go?
There's some documentation links (not specific to applets) on the MATE wiki:
http://wiki.mate-desktop.org/contribute. You could also try the MATE development forum for seeing if anybody has more useful links to share specific to writing MATE applets:
http://forums.mate-desktop.org/viewforu ... 655aa30de1.
Perhaps old GNOME2 applet tutorials will still be of use, though things will have changed from moving GNOME2 to MATE. Here is a "hello world" example as a GNOME2 applet:
http://projects.gnome.org/ORBit2/appletstutorial.html.