The countries that have an Au Pair program are limited to the western countries due to the costs of having an Au Pair and due to the lifestyle of people from those countries.
But this is changing as we get more and more signals that other countries are discovering the immense benefit of exposing you children to a foreign culture and in return make your own culture known to the outside world.
Another fact is that people find different ways to work and travel and they use workarounds to go to countries that have no official Au Pair program.