Serbian language

The Serbian language is a language of Slavic origin from the country Serbia. Serbian is used by the antagonists in Day 1 of 24.

Day 1
The Drazen family is from Serbia, and while talking privately, they often speak Serbian.

When Jack Bauer is fighting Ted Cofell, Cofell curses at him in Serbian. Jack immediately realizes what language he is using, revealing that Jack can recognize the Serbian language.