There's C++ code there too, but that's used by the programmers to expand the game-engine and develop the game. Sure, you could tweak the game-engine and create new stuff for your mod, that's why it's called open-source. The way the modding works, at least throught the interface art->ingame -> stats, it's throught XML files, but also with some Java Scripting, depeding on what you want to do. Those are the two main languages to mod the game. That's what we use. 0.A.D. itself is basically a mod of Pyrogenesis, the engine, which is written in C++ or other stuff that I don't know about.