Assembly, which flavor IDK but some RISC architecture.
If everyone spoke assembly the world would be a very different experience. I'm not saying that it would be better or worse, but it'd definitely be different.
I think the problem is that processors can have slightly different instruction sets, possibly less likely with ARM and RISC (Reduced Instruction, after all), and how they interpret bits and jumps (big endian, low endian). Chibi Akumas has a lot of material for learning assembly of various CPUs, including older ones like 68k and 6502, something I'm doing on and off once in a while