This passage states that in 1822 Vuk Karadžić admitted that he didn’t know if Serbs even had a language of their own and thus had to create one. Bulgarian sources state that Serbs used Old Bulgarian as their language.

Cited:

“All reviewers, even those who have been most critical of this book, admit that it (just as it is now) is of great benefit and importance to the whole world, and they want it to be improved and continued. And besides this general benefit, what benefit does it not bring to our language! Serbs still argue and argue to this day whether they have a language or not;”

Reference

“KARADŽIĆ, Vuk Stefanović. Supplement to the Saint Petersburg Comparative Dictionaries of All Languages and Dialects: with special observations on the Bulgarian language – Vienna, 1822.”