Here are the translations of the greeting "Happy Birthday" to three languages of Yugoslavia:
Croatian - Sretan rođendan
Slovenian - Vse najboljše (Vse najboljše za rojstni dan)
Macedonian - Среќен роденден (Sreken rodenden)
The three official languages of Yugoslavia were Serbo-Croatian, Slovenian, and Macedonian.
Serbo-Croatian has an eastern and a western variant; it is written in the Latin alphabet in Croatia and in the Cyrillic alphabet in Serbia and Montenegro.
Like Serbo-Croatian, Slovenian, which uses the Latin alphabet, became a literary language in the nineteenth century. Macedonian, which has elements of both Bulgarian and Serbo-Croatian, obtained a standardized Cyrillic-based alphabet and orthography only after World War II.
Learn more about Yugoslavia (former) languages at Photius.com.
Tip! Learn how to wish someone a Happy Birthday in many languages at Omniglot.