I assume that the school is an elementary or secondary school (U.S. equivalent of kindergarten through grade 12) based on your reference to an "alloted 10,000 USD a year to contribute to the tuition." To quote IRS pub 970, "[a]n eligible elementary or secondary school is any public, private, or religious school that provides elementary or secondary education (kindergarten through grade 12), as determined under state law." So in order to have the school in Denmark deemed an Eligible Educational Institution for 529 purposes, it sounds like one of the U.S. states (or a similar U.S. jurisdiction) will have to make the proper determination for that school. I think that will be highly unlikely.
https://www.irs.gov/pub/irs-pdf/p970.pdf (see Chapter 8)