Priscus

I hypothesized that Germanic was used as a lingua franca, ie. a
language used where none of the participants in the conversation
speak each other's language.

cf.

Priscus, fr. 8 in Fragmenta Historicorum Graecorum:

"
For the subjects of the Huns, swept together from various lands,
speak, besides their own barbarous tongues, either Hunnic or Gothic,
or--as many as have commercial dealings with the western Romans--
Latin; but none of them easily speak Greek...

After him Zerkon, the Moorish dwarf, entered. ... On the occasion of
the banquet he made his appearance, and threw all except Attila into
fits of unquenchable laughter by his appearance, his dress, his
voice, and his words, which were a confused jumble of Latin, Hunnic,
and Gothic.
"

It is the job of court jesters to tell the king, in a humorous
manner, that which his courtiers are afraid to tell him (cf the
German word "jester's licence"). The jester is here predicting what
the future languages of this part of the world will be: a confused
jumble of Latin, Hunnic, and Gothic. Attila doesn't laugh; he knows
already.

Torsten