:  
 The ballad is generally held to have originated from prevalently Croat Imotska krajina. 

 That particular part of geography was Bosnia at the time, and all the way up to century.] 

 So, if you are going to use geographical origins, it can only be "Bosnian ballad". "Bosnian ballad with oldest account of it written in Croatian dialect" - at best. 
 Hasanaginica is a part of Bosnian culture. Language it's earliest record found today is in, is a bit irrelevant in that regard. It's oldest record could be written in Swahili  - it is still Bosnian. 
 You know... kind of like Robin Hood being part of British culture, not French, despite the fact that his king at the time Richard the Lionheart, or Cœur de Lion ruled over great deal of today's France, or that he as a King of England is buried in Anjou, France.  
 Or that the earliest records of stories and ballads of Robin Hood are not written in today's English. 
 Now... being that we are having this talk in English... I would REALLY love to see what an Englishman or American or German would think of the differences between Croatian, Serbian and Bosnian versions of the song. 
 Is Hasanaginica even thought in Croatian schools today? I KNOW that it is thought in Bosnian schools.  
 That is Croatian as in "Republic of..." not "... people" or "... language". Same goes for Bosnia. 

  
 Oh... and "Osman" has an author. Folk songs don't.  
 Folk living there at the time were of various ethnicity but of Bosnian culture, under Ottoman rule.