What type of language would the subaru "Denso" computer use in my 2002 impreza wrx?

I think it would mostly be an subaru made computer.
Probably you will need c, or c++ if you want to hack something for it. And be prepared to not have much documentation...

System management "computers" made for vehicles are not computers as you would know one.

They are embedded microcontrollers more akin to the PLC than they are to the PC. Programs are normally coded in either in C or C++ as Thierry suggests, and compiled up for the target controller or a controller specific compiler/assembler using symbolic language (mnemonics).

Either way the code you would get from one would be in binary (machine code).