It started on Lisp but moved away.
I'm on mobile right now but if you want I can post some examples later.
But why do you care what my position is, and why does it matter why the OP posted the question?
However, and this answers your second question, I did not understand why this question was important to the OP. I do not know but assume the most important thing for a YC company is the founders, their background, their product and their passion. Whichever programming language they use is of no consequence at all IMHO. (Unless we're talking Brainfuck or Malbolge.)