I guess, you'll have to learn so much more then just C++, most likely assembler too, and a lot (lot lot lot) of tricks.
Everytime i see those multi-minute long demos, with music and animations and see those in 64k (or even 4k intros!!) i'm baffled.
You can use any native compiled language without problems (say, Pascal). You can also use any other language as long as its runtime is small enough.
Most important though, much more so than whether you know C++, is knowing graphics programming. In fact, I've been calling myself a democoder for 10 years now, and I still suck at graphics programming, which is why my demos suck. Focus on the graphics, not the language. A fondness for math is much more important than a fondness for compilers.
I'd recommend making a demo in your favourite programming language. Don't care about size, care about looks. Once you've learned to make it good, you can consider learning, say, C(++) to make it small enough for the 64k or 4k categories.
Sidenote: most 4ks these days are entirely coded in a shader. This means you can mostly copy&paste the entire C part from somewhere, and start hacking GLSL or HLSL. Sure, you'd need to learn the shader language, but it's smaller and simpler than c++.
