It shouldn't take too long to get to a point where you can make a useful contribution to 0 A.D. by learning JavaScript. This tutorial will give you a good start, although as with most JavaScript tutorials it focuses on scripting for the web. Still, it should give you a good understanding of the language and from there you can start taking a look through the JavaScript files used by 0 A.D. - make small modifications, run the game, see what changes, rinse and repeat.