The language is very hard to define whether it belongs to Thraco-Illyrian or to Hellenic groups of Indo-European languages. Some linguists believe that tribes of mountainous Macedonia spoke an archaic Thraco-Illyrian language, and people in towns and the upper classes, influenced by Greek achievements, gradually were losing their native tongue and took up Greek. Contacts with Greek Halkidiki and Thessalia regions were strengthening in the 5th and 4th centuries, and simultaneously the process of national assimilation went. And when Greece was conquered by Philip and occupied by his son Alexander the Great, Macedonians officially became real Hellenes.
Anyway, original Macedonian had much in common with Greek in structure and morphology. Its dictionary consisted of words cognate with Thracian, Greek and Illyrian ones, and some Macedonian words were borrowed by Greeks. Nowadays the lands of ancient Macedonia are divided between the Republic of Macedonia (inhabited by South Slavs), Greece and Bulgaria (also Slavic). And another language, a Slavic one, is now called Macedonian. But it's completely different, though Greeks still oppose the use of the name "Macedonia" to refer to the former Yugoslav republic, and they object as well to calling the language of that region "Macedonian".