*Not exactly. The affirmative (sort of) head gesture in Bulgaria and India (not just Sri Lanka, but India) is not identical with the straight horizontal side-to-side negative headshake. It’s a more complex motion as if the crown of the skull is describing a figure 8. It has a rounded motion combining the horizontal, vertical, and slanted in between. The impression you get is “maybe yes, maybe no, I’m not committing myself either way.”
The negative head gesture (at least in South India and southern Italy, two places I’ve been) is not the same as the affirmative up-and-down-and-up nod. It’s a single sharp jerk of the chin upwards, generally accompanied by a tongue click. This gesture is too abrupt to be polite. A more genteel, evasive way of conveying the negative is to lift the hands in a helpless little shrug. This is especially done in the Mediterranean basin and the Middle East where indirection is such an important part of getting along with others.
The odd thing, in the part of South India that Mrs. Mojo comes from, is how they say “uh-huh!” to mean “No way!” Could be very confusing to a non-Indian, taking it to mean the opposite.
In the miniscule bit of Chinese I’ve learned, the word that’s equivalent to “yes” is not the usual means of affirmation. Instead the question is posed this way: “X (not X)?” To affirm, you just repeat “X”. To negate you say “not X.” (Is this where symbolic logic came from?) Example: Ni dong le ma ‘You understand?’ or Ni dong le bu dong le ‘You understand / not understand’. To say “yes” you would answer dong le; to say “no” would be bu dong le.
How to give a clear unambiguous answer to a negative question? Languages other than English and Chinese are better equipped for this.
In French, to answer a negative question, you can’t use oui. If the answer is really negative, you say non. But if the answer is positive, contradicting the negative, you have to say si. Tu n’as pas compris? ‘Didn’t you understand’? If you really did understand, the answer is "si."
In Arabic, the regular word for ‘yes’ is na‘am. But the positive answer to a negative question is balâ. hal lâ tafham ‘don’t you understand?’ If you don’t understand, say lâ ‘no’; if you do understand, say balâ ‘On the contrary, I sure do’. You also use balâ to contradict a negative statement. —hâdhihi al-tuffâhah laysat yâni‘ah ‘This apple isn’t ripe’. —balâ ‘Oh yes it is!’
As Terminus Est (can I call you Tetelestai?) noted above, English yes was once this very useful word to oppose a positive to a negative. To remove ambiguity from the usual yea. “Art thou in sooth he whom they call Wat Tyler?” “Yea, 'tis I.” A simple yea used to suffice with a positive question.
But: “How now, varlet, hast thou not swept the stables?” “YES, good my liege, nigh upon an hour ago.”
By losing the distinction, our speech now carries less information than it used to. Allowing for confusions like “Yes, we have no bananas” and Chinese mixups. In Navajo (which shares several similarities with Chinese) they likewise give a negative answer with “Yes.”
The simple Anglo-Saxon yea has not died out in the present day: people still say “yeah” but it’s considered substandard speech. Maybe while it survives there’s a chance that the distinction between the two affirmatives can be revived (but only if speakers feel a need to do so—maybe with more Chinese coming here…).
