Simple answer: Gothic (which was a Teutonic language).

But the initial question has to be which Goths we are talking about.

Let’s listen to Jordanes talk about the Gothic conquests:

“Soon Geberich, king of the Goths, departed from human affairs and Hermanaric, noblest of the Amali, succeeded to the throne. He subdued many warlike peoples of the north and made them obey his laws, and some of our ancestors have justly compared him to Alexander the Great. Among the tribes he conquered were the Golthescytha, Thiudos, Inaunxis, Vasinabroncae, Merens, Mordens, Imniscaris, Rogas, Tadzans, Athaul, Navego, Bubegenae and Coldae. But though famous for his conquest of so many races, he gave himself no rest until he had slain some in battle and then reduced to his sway the remainder of the tribe of the Heruli, whose chief was Alaric. Now the aforesaid race, as the historian Ablabius tells us, dwelt near Lake Maeotis in swampy places which the Greeks call hele; hence they were named Heluri. They were a people swift of foot, and on that account were the more swollen with pride, for there was at that time no race that did not choose from them its light-armed troops for battle. But though their quickness often saved them from others who made war upon them, yet they were overthrown by the slowness and steadiness of the Goths; and the lot of fortune brought it to pass that they, as well as the other tribes, had to serve Hermanaric, king of the Getae. After the slaughter of the Heruli, Hermanaric also took arms against the Venethi. This people, though despised in war, was strong in numbers and tried to resist him. But a multitude of cowards is of no avail, particularly when God permits an armed multitude to attack them. These people, as we started to say at the beginning of our account or catalogue of nations, though off-shoots from one stock, have now three names, that is, Venethi, Antes and Sclaveni. Though they now rage in war far and wide, in punishment for our sins, yet at that time they were all obedient to Hermanaric’s commands. This ruler also subdued by his wisdom and might the race of the Aesti, who dwell on the farthest shore of the German Ocean, and ruled all the nations of Scythia and Germany by his own prowess alone.”

What was the lingua franca of this Gothic kingdom?

Now, let’s turn to Ammianus Marcellinus and his discussion of the conquests of the Alans, neighbors to the Goths:

“On the other side of this river [Tanais/Don] the Halani, so called from the mountain range of the same name, inhabit the measureless wastes of Scythia; and by repeated victories they gradually wore down the peoples whom they met and like the Persians incorporated them under their own national name. Among these the Nervii inhabit the interior of the country near the lofty, precipitous peaks nipped by the north winds and benumbed with ice and snow. Behind these are the Vidini and the Geloni, exceedingly savage races, who strip the skins from their slain enemies to make clothing for themselves and coverings for their horses in war. On the frontier of the Geloni are the Agathyrsi, who checker their bodies and dye their hair with a blue colour — the common people with a few small marks, but the nobles with more and broader spots of dye. Beyond these are the Melanchlaenae and the Anthropophagi, who according to report lead a nomadic life and feed upon human flesh; and because of this abominable food they are left to themselves and all their former neighbours have moved to distant parts of the earth. And so the entire north-eastern tract, until one comes to the Seres, has remained uninhabitable. In another part of the country, near the abodes of the Amazons, the Halani mount to the eastward, divided into populous and extensive nations; these reach as far as Asia, and, as I have heard, stretch all the way to the river Ganges, which flows through the territories of India and empties into the southern ocean. Thus the Halani (whose various people it is unnecessary now to enumerate) are divided between the two parts of the earth, but although widely separated from each other and roaming over vast tracts, as Nomads do, yet in the course of time they have united under one name, and are, for short, all called Halani because of the similarity in their customs, their savage mode of life, and their weapons. “

Even assuming many of these are concoctions of a man schooled in Herodotus and the classics, the message of an Alanic multi-ethnic “empire” is unmistakable.

And then what happened as the Huns attacked?

“The Huns, then, having overrun the territories of those Halani (bordering on the Greuthungi) to whom usage has given the surname Tanaïtes, killed and plundered many of them, and joined the survivors to themselves in a treaty of alliance; then in company with these they made the more boldly a sudden inroad into the extensive and rich cantons of Ermanaric… After his demise Vithimiris was made king and resisted the Halani for a time, relying on other Huns, whom he had paid to take his side. But after many defeats which he sustained, he was overcome by force of arms and died in battle. In the name of his little son, Viderichus, the management of affairs was undertaken by Alatheus and Saphrax, experienced generals known for their courage…During these days also Vithericus, king of the Greuthungi, accompanied by Alatheus and Saphrax, by whose will he was ruled, and also by Farnobius, coming near to the banks of the Danube, hastily sent envoys and besought the emperor that he might be received with like kindness.” 

Now, Alatheus, probably, and certainly Saphrax, are not Gothic names. Vithimiris is most likely a Suavic name even if Viderichus is Germanic. Farnobius is likely neither.

Once the Huns and Alans overcame the Ostrogoths this whole menagerie of nations settled in Pannonia. What was the relative size of the various populations?

Well, we know both from Jordanes and from Ptolemy (and Tacitus to some extent) that the Veneti were a multitude or roamed vast distances.

So what language did these people assembled under Attila speak?

Let’s turn to Priscus (via Jordanes):

“instead of grain we were given millet; instead of wine, a drink grown locally called medos. The servants following us also brought millet and supplied a drink made from barley, which the barbarians call kamon.”

And some more from Jordanes:

“When the Huns had mourned him [Attila] with such lamentations, a strava, as they call it, was celebrated over his tomb with great revelling.”

Otto Mänchen-Helfen did not mince words when he wrote that Vasmer’s (and Schwartz’) objection to a Suavic etymology of strava (should have been sutrava) “cannot be amen taken seriously.” He also pointed to Popovic’ (in Sbornik Radova vizantoloshkog instituta 7 (1961)) and, earlier, Kotliarevskii’s (1863), Nehring’s (1917) and Trautmann’s (1944) support for the Suavic etymology.

Mänchen-Helfen also concluded that kamon/kamos was the earlier Pannonian kamum (citing Julius’ Africanus’ “Embroideries” and Diocletian’s Edictum de Pretiis) and that medos was either Germanic or Illyric. Both of these words also have Suavic cognates (or maybe etymologies?).

As another side note, Mänchen-Helfen also proposed that the Sclavenes/Venethi reference in Getica that mentions the Vistula (as opposed to Viscla) was the original Cassiodoran version (not Jordanes’). Notably that is the passage that speaks of the Venethi possessing vast swaths of territory seemingly north of the sources of the Vistula.

But let’s get back to the argument:

Who fought the Goths after the demise of the Huns? Jordanes tells us Skiri, Suavi, Sarmatians, Gepids and Rugi – another motley crew of nations. What language did they communicate in?

As already noted, Thursday was labeled the “fifth” day in Gothic (as reconstructed *pintadags or *paintedags. Again, compare this with the Polish piątek – meaning “Friday”. And again, Friday is reconstructed as *pareinsdags (but also paraskaiwe) which suggests a curious similarity to Piorun/Perun. Compare this with the Polabian Perĕndan for Thursday (Thor’s day).

And there are many other words in Suavic that ring of Gothic-Suavic connections: gniew (“anger” but compare with “knife” and Gniew(ko) with Cniva) or sługa (notwithstanding Brueckner, > słuchać?).

It seems to me then that it was the Goths, at first on their own and then driven by the Huns that spread the Venetic, that is, Suavic, population around much of the Balkans. It would not therefore surprise me that the vast majority of the “Goths” (as in those people conquered by the Goths and who served in the Goths’ armies) actually spoke the Suavic lingua franca.