I don't know if there is an online dictionary but here is a freeware French-Serbian dictionary to download. It has 4,681 Serbian->French entries and 4,013 French->Serbian entries, which is not much but still better than none. I can't speak Serbian so I couldn't test its quality.
Hope this helps...
PS - You have to download the program ("télécharger le programme") AND the vocabulary list ("télécharger la liste").