Linguistic evidence now tenuously points to ancient Macedonian language as related to North-West Dorian Greek (a cousin to Locrian, Epirote, and Aetolian). Actual evidence of the language is pretty slim, but many prominent scholars have begun to tentatively support this in recent decades (including Hatzopoulos and Meier-Brügger). This is based largely on the discovery of a lead curse tablet at Pella in 1986, which is one of a kind and is written in a version of North-West Doric Greek. Culturally, however, especially in the pre-Classical period, Macedonia shares more with Thracian practice. Their social organization was predominantly tribal rather than centered on the polis, and shares many features common in the elite-based horse cultures to their north and east. For instance: they were ruled by a king, whose position was nominally hereditary but also subject to the "election" of a narrow tribal aristocracy. Succession always seems to have been a violent, anxious affair, even after Alexander.

By the 6th-5th century, Macedonian elite were speaking Greek and had adopted most of the socio-cultural trappings of the Greek city-states. The points of influence are obvious, and there were Greek polis enclaves along the coasts of Macedonia and on the adjacent Chalkidike since Archaic times.

So: a people with a cultural sub-stratum of Thracian and/or Illyrian tribalism, but with a language which is probably directly related to the Hellenic branch in some way, probably via North-West doric, but quite dissimilar to the Greek known from the great city-states of mainland Greece. The Macedonian language had largely been replaced by Greek by the Classical period, especially among the elite, who tend to leave the most substantial records behind (civic and funerary inscriptions, e.g.). Culturally, the Macedonians are adopting more and more "Greek" elements in the centuries and decades leading up to the reign of Philip II, including participation at pan-Hellenic sanctuaries, but Macedonian social organization remained primarily tribal, with something like a hereditary monarchy with (debatably weak) hegemony over the whole area. This compares sharply to the Greek development of many city-states, each with a small territory (chora). The general impression we get is of a "crossroads people" who were at the intersection of city-states and tribal aristocracies, of Greek and non-Greek, with elements of both.

There is a brand new monograph in the excellent Routledge series: Carol King, Ancient Macedonia (Routledge, 2017/18), the first new English-language work on ancient Macedonia in several decades. It's very good. If you are unable to access it from your institution (or don't have an institution and want to read it), let me know privately and I can help you get access.