They would only be able to seize assets you might have left in the US. Extradition is only for criminal cases.
Whether or not a country will assist at all in enforcing a civil judgment from a foreign jurisdiction is a matter to be explicitly spelled out in treaty agreements.
Your best bet for avoiding a civil judgment is probably to just be too difficult for a process server to find to serve you with the complaint and summons, or failing that, to make a special appearance to assert lack of jurisdiction before making your answer.
IANAL. While I'm not particularly inclined to offer helpful advice to those who would try to skip out on valid debts, I am even less inclined to allow the typical institutional lender or debt collector to go about their business as usual and win by default.