Translation:No, they don't
Quite a few :) Because it depends on the person ("No, I don't ..." vs. "No, we don't ...") and tense ("No, I don't ..." vs. "No, I didn't ...") and possibly also on the verb - many sentences are formed with bod "to be" and so the "no" answers will use forms of that verb, but if you use a different verb such as gwneud "to do", the "no" answer can be different.
From being around welsh speakers a lot, they give up with all the different variations and just say "nah" but you should know when to use the different variations anyway.
Actually, this occurs in some other languages - no equivalent of just universal No, for example - Mandarin Chinese.
Thanks Mizinamo, but is there a web site that you know of where I could study this in a bit more detail?
Here's a PDF that may be useful in this respect: http://www.bbc.co.uk/wales/learnwelsh/pdf/welshgrammar_ff_replies.pdf