For example:
"Kyle Bass Is So Bearish On Japan, He Financed His Home In Yen"
"The Dallas hedge fund manager (no relation to the famous Bass family of Fort Worth) is so convinced the Japanese government's profligate spending will drive the nation to the brink of default that he financed his home with a five-year loan denominated in yen, which he hopes will be cheaper to pay back than dollars."
This bet may have paid off. The Yen/USD went from 80-90 to 1 in 2010, to 122 to 1 by mid 2015 with the Japanese move to debase.
https://www.businessinsider.com/kyle-bass-is-so-bearish-on-j...
Swapping loans is fairly trivial, and would leave one with a mortgage rate competitive with American ones.
Assuming they make money, could they then go sub zero?
The loans are considered assets of the bank, so still increases it's value. And if the loan ends up in an account on their books (there's not that many banks in Denmark), they can then loan out that money again minus the fraction they need to hold.