Slander is slander no matter where is shows up.

If the negative speech constitutes defamation (more specifically libel), then yes. The foreign person could sue you in America. If the speech does not create a cause of action in America, but does create a cause of action in another country, then you can be sued in that other country. However, American courts will not enforce the judgement against you if your words would have been protected by the 1st Amendment and if the words were posted on a server located in America. Perhaps a real life case will help you understand. Yahoo was sued in a French court because its U.S. site contained Nazi paraphanelia. The display or sale of Nazi paraphanelia is illegal in France. The French court sided against Yahoo and awarded damages. The plaintiff came to America to have the judgement enforced (which was possible due to a treaty to which the US is a signatory). The US court refused to give full faith and credit because the order violated the 1st Amendment, and therefore, was against US public policy. You should note that the French version of Yahoo did not have the Nazi paraphanelia, and the case may have been different if it had.