I learned Applesoft BASIC to draw a surprise Dickbutt.
If we're counting machine code, I learned 6502 ASM for faster division on NES, because it was half the CPU time on my first-person shooter. After many iterations pushing it down to mere hundreds of cycles, I slapped my forehead and implemented log tables in like 512 bytes and 45 cycles. It's negligible now. And supports constant fractional scaling. And has overflow / underflow saturation. Really, 6502 ASM is fantastic to fuck around in, even though the rest of the NES's hardware suuucks.