I'll start with a joke. I've heard it in Greek, but it works in English too.
A linguist is making a speech about languages.
There are languages, he says, where a double negative produces an affirmative.
In others, a double negative still produces a negative.
However there seems to be no language where two affirmatives produce a negative.
And someone from the audience shouts: "Yeah, right!"
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I would think all languages have a word for yes and no (though in Japanese you learn about them rather late, as they are not to be overused).
A very common little word that I find fascinating is the German "doch". Doch means yes, even though you were expecting a no. What a nice, practical concept!
A dialog would go like this.
-There is nothing interesting on the internet.
-Doch, check out MR for example.
