Here's a preliminary view (I have yet to consult Pachkova's latest /2006/ opus on the Zarubinian culture, but have incorporated conclusions mentioned in passing in other articles). I will concentrate on the northern Bastarnae (i.e. Zarubinia) for the moment, since they are now considered to have been the major players in the Balkan campaigns of 179-168 BCE.
The linguistic evidence, such as it is, is twofold: (a) documentary, and (b) topo/hydronymic. The archaeological evidence is somewhat helpful when coordinated with (a) and (b). All is very tentative of course, but the hypothesis has a strong level of plausibility.
1. Re (a) Strabo's account (7,3,17) is almost certainly drawn from a source which discussed the Bastarnae of Mithradates' time frame (i.e. first half of the 1rst c. BCE), since he admits he has no recent knowledge of them. The Peucini might, however, be an exception to this (after all they were a border people for areas of Roman interests), and so his tentative mention of the B. as within the "Germanic" fold might refer to that particular group. This would then be in line with (a)- type documentation from the 1rst c. CE (Pliny and Tacitus). But Strabo's information about the Bastarnae of the "interior" would be, possibly, less certain for his time.
2. Earlier Greek authors (e.g. Polybius, and Livy's source for the 168 BCE events) counted the B. among the "Galatae", which may or may not mean Celts. We know that the prince of these northern Bastarnae, throughout the Balkan campaigns) was one Clondicus. And this sounds Germanic (for the Greeks the term "Galatae" would also cover this group at the time). Livy's source states that these Bastarnae could communicate with the Scordisci. At first (and perhaps second) glance, this might suggest that there were important Celts among the leadership. But there is one complication: the Scordisci were a "mixed" group, Celto-Illyrian and/or Celto- Thracian. The language of Bastarnian-Scordiscan communication could hardly have been Germanic (proto- pre- or whatever) in 179 BCE. It could have been Celtic, but it could also have been Illyrian, or (perhaps less likely) "Thracian". The case here
rests on (b)-type documentation. The topo/hydronyms of northern Bastarnia are (today) solidly Slavic. But for the period prior to 500 CE, investigators have also discovered Baltic, Celtic, Germanic, "Illyrian", and also "Thracian" remnants. By "Illyrian" BTW Ukrainian linguists mean not only those of the Balkans but also the population from the area of the old Lusatian culture in Poland (sometimes also called "Veneti") which they believe to be linguistically related. So the Livy evidence is ambiguous, and remains so even after coordinations with archaeological evidence.
3. The archaeological evidence shows that the northern Bastarnae were divided into three distinct groups. There were important "sharing" characteristics (esp. in a part of their ceramics, and in their fibulae inventory and burial setups), but there were equally significant distinctions, which archaeologists believe to have been the result of distinct substrate elements (more or less powerful) in the three areas. The "incoming migrants" were themselves of two archaeological categories: Late Pomeranian (=Venetic or "Illyrian", but also Celtic.NB: there is some but very little "pure La Tene" east of the Carpathians, so the explanation of the Celtic topo/hydronyms is today that incoming Celts were among the Late Pomeranian elements) and Yastorfers (=Germanics). These migrants were not equally distributed in all three areas either as a whole re the "locals", or with respect to each
other. Also: there was no territorial continuity between the three groups, i.e. large areas of archaeological "nothingness" extended between the territories of group1, group 2, and group 3. The economic relationships of the three groups also differed significantly. And the historical fate of the groups after 50 BCE and esp. after 50 CE were different. Group 1 was located south of the Prypjat' (Pripet) river, straddling today's border between Ukraine and Belarus in the area of the Horyn and Styr. Group 2 extended along the Middle Dnipro, from north of Kyiv to south of the Ros' river and even further almost to the steppes. Group 3 was located on the upper Dnipro between the Sozh and the Berezina, in eastern Belarus.
4. Before the arrival of the western "migrants" the territory of group 1 was inhabited by representatives of the Lusatians (with a first wave of Late Pomeranians settling in the 4th c.) in the west, and Milohradians (a Baltic unit) in the east. The territory of group 2 was shared by Pidhirtsovians (the Ukrainian label for Milohradians) in the northern section above Kyiv, and "Scythians" in the south. These "Scythians" were likely a Thrakoid group whose aristocracy had been largely incorporated into the Aukhata ("agricultural Scythians") in classical Scythian times. The territory of group 3 was completely "Baltic" but under some "Scythian" influence from the south. These were the "local" substrate populations of northern Bastarnia.
5. The migrants (Late Pomeranians and Yastorfers) settled more densely on the territory of group 1, somewhat less so on that of group 2 (archaeologists estimate that there were three times as many "migrants" among the population of group 1, relative to the whole population of the group, as among that of group 2. Very few "migrants" settled in the group 3 area, and the Zarubinian culture spread there apparently by voluntary "local" acculturation and by some movement northward from group 2 "acculturated" ex-"Scythian" elements. The "migrants" constituted approximately 25% of the total population of group 1 (as reflected in the examined archaeological sites) , and perhaps 10% of that of group 2. Their social power was clearly much greater than their numbers, since it is their culture which created the key distinguishing characteristics of "Zarubinia" esp. in "prestige" ceramics, funerary patterns and "aristocratic" apparel
(esp. fibulae). It's difficult, of course, to estimate language use in such societies, but it seems plausible to postulate long epochs of stable multi-lingualism. Besides Celtic, Germanic, "Illyrian", "Thracian", and Baltic (or BaltoSlavic?), Iranic and Greek might also have been languages of use among some sections of the population. Iranic in group 2 which had strong economic ties with Scythia to the south esp. the Lower Dnipro cities (there was a colony of Zarubinians there and a colony of Scythians later settled in a northern fortress near Kyiv). North Zarubinia also continued to be an area of attraction for migrants from the west: newer Yastorf (or already Elbe Germanic), Przeworsk, and even Oksywie elements have been found among group 1 and 2 locations for the first c. BCE. And in the 2nd c. there will be substantial Przeworsk in- migration into group 2 locales. Again it's hard to estimate. but it seems that a case can be made for there being a
little more Celto-Illyrians than Germanics in the north (numbers wise) and the reverse in the south (in the Poeneshti-Lukashovka area). In the phase 200-50 BCE this made no discernible difference.
(to be continued)