ATP Conversion
"Little Bubba" is right, but individual country requirements vary for converting your U.S. ATP certificate to their local ATP.
For example, I recall reading on either the Australian or New Zealand Ministry of Transport site that you have to take as many as two writtens and a flight test or local BFR to convert your U.S. certificate.
Try looking for a particular country's aviation regulation body and you'll get more specific information.
Hope this helps a little.