Odds and Ends
Welsh has several words for 'yes', but doesn'
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Not exactly, as those are adding a first-person subject, which changes the meaning - you're not just answering "no" anymore, you're now specifying that you're personally in disagreement.
"Nac" (often mutated to "nag" depending on the following word - Welsh does a lot of mutations, it's a pain) - doesn't really get used by itself, but it's used in conjunction with specific other words to negate their meaning. So Welsh speakers do literally say "not yes".