Ancient or modern Macedonian is the language of the ancient or modern Macedonians of the Greek Kingdom of Macedonia and now the Macedonia region of Greece. Since antiquity up to date the language it is spoken. The variety of Doric Greek was spoken all across the Ancient Kingdom, influenced later by the Attic Greek variety. During the first Greek Empire, the Hellenistic period, Greek language varieties formed the Koine Greek which it means "Common Greek". The language evolved and simplified only in the past few centuries to modern Greek. Since the ancient times, different varieties of Greek are usually in the type of idiom or different accent. Ancient Greeks, either they are from Athens, Sparta, Macedonia, Epirus, Ionia or elsewhere, they understood complete each other even when they speak different variety of Greek. The same can apply today. Also modern Greeks could clearly understood ancient or koine Greek and the same would work for the ancients either.
The Ancient Macedonians were speaking Greek. Athenians, Spartans, Epirots, Ionians, Thebeans and many more, same with Macedonians, they were all speaking Greek, they all believed in same Gods, the Olympians, and all of them consider themselves Greeks.
The surviving public and private inscriptions found in Macedonia indicate that there was no other written language in ancient Macedonia but Ancient Greek, and recent epigraphic discoveries in Macedonia, such as the Pella tablet, suggest that ancient Macedonian have been a Doric Greek.