Why the heck do English speakers say 'yes' for the affirmative?

Just another data point: In classical Latin, there is no word for “yes” or “no”: You answer a question by repeating the verb. For instance: “Do you swim?” “I swim”. Of course, the Latin word for “I swim” is… You guessed it… “no”.

In English, nay means ‘no’, but in Modern Greek nai means ‘yes’ and in Korean ne means ‘yes’.

In Polish, tak means ‘yes’ but in Malay tak means ‘no’.

Isn’t the Greek word for “no” pronounced oh-kee? Or is there a colloquial alternative?

Things can get hairy in Greece if you aren’t paying attention. Okie, nai, no, yes???

Also, I’ve read that in Greece, smiling can be taken as a sign of anger. Any old smile, or a specific type of smile (like a beauty-queen forced smile)?

That’s right, sjc. To answer a “yes or no” question you use either the positive or the negative form of the verb used in the question (in Irish, in most cases this simply involves repeating the verb - positive - or preceding it with - negative). This is also the case in the other Celtic languages, though it is by no means unique to those languages.

Bordelond, nai, I mean yes, the Modern Greek for ‘no’ is okhi. You might find some Greeks telling you it’s “ohi,” depending on their dialect.

ruadh, so the Irish way of negating and affirming is the same as in Chinese?

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sjc to answer your question about Irish Gaelic not having a yes or no. They do not. I asked the same question in this thread here.
http://boards.straightdope.com/sdmb/showthread.php?threadid=51230

Also, several other languages don’t either like Latin as Chronos explained earlier.

Is ea. Also the same as Swahili and Finnish, and several other, smaller languages. It really isn’t as strange as it might seem.