But if the lender sues in a US court and gets a judgement, isn't the person subject to an extradition? The lender doesn't have to sue in respective country's court.
Wikipedia also has a list of countries that the United States doesn't have an extradition treaty with. https://en.wikipedia.org/wiki/Extradition_law_in_the_United_.... Most of those don't look like great options, but a few of them are quite nice!
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.
There are countries that don't have extradition treaties with the US for anything, though many of them are places you might not want to live...