in what are the touhou project games written

C?

Not that if it's good for ZUN is good for me, but I'm thinking of making a shoot them up-style game and I'm curious

Even a cursory attempt to disassemble the binaries would reveal the answer. Sepples and DirectX, with an internal scripting language for the danmaku. Now fuck off, but not before noting that you are going to fail to produce even a single line of productive code, incompetent shithead.