This is why I took Electrical and Computer engineering. They don't screw around with Java\Python, we go straight to VHDL, C, C++, assembly. Which really are the only useful languages to a Computer engineer. I mean, you won't be writing drivers\kernels in Python or Java any time soon (not impossible to do though). Arguably, python is very useful for prototyping a design.
That said, because the code is written to be placed on microcontrollers\FPGAs\PLDs, it is usually _REALLY_ low-level code.
I would argue that C# is multi-platform at this point (technically it always has been.) Most C# projects release their software and link to both the MS .Net framework and mono. Mono has become really quite popular.
For something to be single-platform, it has to be compiled for specifically that platform. We all know that .Net applications are (typically) compiled into bytecode and then JIT assembled to code native to the platform it resides to.