Language: as far as the program? I think it's C++. Modules have been in various languages.
However, forget about the module coding for now. Theming does not require anything of the sort. The configuration files are plain old text files (all OTS2 themes will start out with a theme.rc in the theme's main folder). Remember that documentation is your friend, and so is LSXCommand. Also, just download other themes to mess with, as Austerity is not simple by any means.
Freedom would be a good one to start with. If you can spare the brain power and time, there is quite a bit of functionality available right now, especially with xLabel.