David Dobkin, the movie director, explained in a Vulture interview:

The song-along sequence had been written into Will Ferrell and Andrew Steele’s script with a note about possibly trying to get former contestants to also participate. Dobkin initially thought of getting past Eurovision winners ABBA and Céline Dion to perform their own songs, though they declined. Though the sequence might recall a similar scene from the movie Pitch Perfect, Dobkin was thinking more along the lines of the popular mash-up of Linkin Park and Jay-Z’s “Numb/Encore.” “When you hear that version of those songs, you actually have a hard time pulling apart what’s what and what used to exist where. I wanted a mash-up and a remix dance version of something that you could put in fucking dance club and everybody would love.” He says his wife suggested using Cher’s “Believe,” which became the backbone to the medley.

Again, however, it took a while to get it right. “A bunch of people took a swing at this thing,” Dobkin says. “And I was feeling like I wasn’t expressing what I wanted. It was supposed to be easy. All these versions that were coming up were too complicated.” To help realize the medley, Netflix director of music Amy Dunning connected the director with musician and producer Alana Da Fonseca, who Dobkin says ultimately put together a track for him in 30 minutes that was nearly perfect. “I walked out of there with it on my phone, played it in my car, called her up, and said, ‘This section is too long.’ By the time I got home, she had edited it and it’s what you hear in the movie.” Producing the actual song, of course, was a nightmare. “There are so many people from so many different countries, and getting each of them in the recording studio and succeeding at that was a whole other thing!”