C++ because of its ability to be relatively small in deployment, and have very low level (eg. direct) control over hardware functions. So Cimplicity that is mentioned in that other comment is C based.
That looks more like a full system software solution. I mean like, when you press the 1 button on your microwave and then the display reads 00:01. Or when you press the popcorn button and the microwave starts running for a predetmined amount of time. What is the language that is used to interpret the button presses and send signals to the various components? I always figured that is what "microwave oven programming" entails.