Most of the game was/is written in C++, shaders are written in HLSL (Cg) or Shader Assembly, apparently they also use PERL for some aspects of shaders (last time I checked they had not finished the section on creating shaders). There may be a few different languages used here and there for editors and tools (Max plugin is in Max Script, Maya is probably in MEL, XSI is probably C++ but could be something else, PERL may be used for some file parsing). For the most part though the game is written in C++.