I’d go with graphical animations of corresponding gestures.
In most cultures this would probably be a head shake and a nod:
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Shaking to indicate "no" is widespread, and appears in a large number of diverse cultural and linguistic groups. Areas in which head shaking generally takes this meaning include Indian subcontinent, Middle East, Africa, Southeast Asia, Western Europe, and North America.
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Nodding to indicate "yes" is widespread, and appears in a large number of diverse cultural and linguistic groups. Areas in which nodding generally takes this meaning include the Indian subcontinent (note that the head bobble also shows agreement there), Iran, Southeast Asia, Western Europe, Latin America and North America.
For cultures with different meanings for head shake/nod, you’ll have to adjust it accordingly. Of course this is only possible if you know the background of your visitors. If you expect visitors from many different cultures, you could start with an opening question like "Which gesture represents yes for you?" and let your visitors choose.
Not the best example, but you get the idea, right?
