Depends on the system.

For the early consoles, atari 2600 type, maybe even nintendo, probably a range of assembler to C with lots of assembler. But most of the new stuff is probably in C/C++. If you're curious about a particular platform, like xBox or Playstation (I'll betcha it's C++) some of us in the office know some game coders and we might be able to ask them specifics