How do you say merry Christmas in Yugoslavia?

There is no language or country called Yugoslavia.

Yugoslavia was a country located in southeastern Europe that existed from 1918 to 1992. It was composed of six constituent republics: Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, and Slovenia.

The official languages of Yugoslavia were Serbo-Croatian, Slovene, and Macedonian. In Serbo-Croatian, the phrase for "Merry Christmas" is "Sretan Božić". In Slovene, it is "Vesel božič". And in Macedonian, it is "Среќен Божик" (Sreḱen Božik).