Unfortunately, the second half of Jussive's message is wrong. It should be:
Didn't you eat your breakfast this morning?
No =
No, I didn't eat it.
Yes =
Yes, I did eat it.
Just remember that "No" simply confirms the negative fact that "I didn't eat it."
Also remember, as Jussive correctly said, that positive goes with positive, and negative with negative,
but this refers to whole of the answer. "Yes" accompanies the rest of the positive answer: i.e., that "I did eat it."
Because of this confusion, I totally agree with Jussive that it's best to clarify your yes/no answer with "I did" / "I didn't".
Here's the explanation for this confusion: We use negative questions in English for 3 reasons:
1) We're surprised.
Wow, I'm surprised to see your breakfast still sitting there. ~ (Why) didn't you eat it? (We're expecting some sort of explanation.)
2) We're excited about something and we're making an exclamation.
Gee, doesn't that dress look nice! ~ Yes, it sure does! OR No, I don't think so. It's totally out of date.
3) We expect somebody to agree with us.
Who doesn't live in Spain? = No-one. Nobody lives anywhere else; it's not cool to live anywhere else. Everybody lives in Spain.
In the case of Velfarre's original question, the first word, "Who" (instead of the negative verb) would be stressed. The questioner wants to know who the person is who doesn't live in Spain.
Who doesn't live in Spain? This case carries no surprise, excitement or expected agreement. I would answer "I don't." or "I'm the one who doesn't live in Spain." or "We're the ones who don't live in Spain." To figure out your answer, think of the complete sentence. "Most English people don't. (live in Spain)."
(I'm an English-As-A-Second-Language teacher in Canada.)