If someone moves to another country (other than the U.S.), can his wages be garnished to collect student loan debt?

It depends on which country you're talking about. Most developed countries are signatories to international agreements that enable collection of debt even across international borders. Note also that if you try this, and default on your loans in the U.S., a judgment entered against you will continue to exist and accrue interest here. You would be subject to collections efforts in the U.S. and other signatory countries forever. This seems like a poor way to get out of your financial obligations. You should instead look into loan consolidation and income based repayment. Check out https://www.mohela.com/ .

Not very likely. You can contact the court where the debtor is located and ask them whether or not they will execute a foreign judgment. I hope you speak their language.