Born in England to a Danish-German father and a Bosnian mother, Biereth had the chance to represent four countries at the international stage: England, Germany, Denmark, and Bosnia and Herzegovina. However, he chose to pledge allegiance to Denmark from a very young age due to his father.
In a 2023 interview with Danish publication Bold, Biereth, who was at the time a part of the Denmark under-21 setup, explained that his father played a huge hand in him becoming a footballer.
"I think it has something to do with my relationship with my father. He was the one who got me into football in the first place. So it's probably about making him and that side of the family proud," he said.
The Monaco forward also made it very clear that he never considered representing any other country, and that he won't change his decision in the future: "But for me it wasn't like I wanted to play for Bosnia, Germany or England. I wasn't drawn to them. So I feel like I've made the right decision now. There's no chance I'll change it. It's my final decision."