It is still too young to be considered a folk-song, it dates from the WW1, but in 100 years more, one will perhaps call it that way.
Here a nice Serbian folk song with Russian subtitles. For more on Serbian traditional music, visit this thread.
P.S. There are several threads on the same topic, I believe I opened one (Sounds of Slavia), as well as Dervan did sometime ago before it (National music). This thread from Svätoslava, could also fall under the same category (Folk songs with interesting lyrics). One may merge them so we would have everything under one thread, and not multipost.