I am currently listening to the excellent "History of Rome" podcast and one thing that struck me is how many different peoples played crucial roles in the diplomatic and military matters facing the roman empire in the 4th and 5th century. What I'm wondering is how the romans made deals with hunnic kings, frankish, gothic, vandal, etc. chieftains, berber tribes and the persians.
Of course, long-standing contact with peoples like the franks and of course the persians will have given them ample opportunity to learn the language, but how did they broker deals with the huns, who they had almost no direct contact with before? While I imagine that greek and latin were widespread as diplomatic languages even outside the empire, it still boggles my mind how communication across these linguistic and cultural gulfs was handled in practice.
Language-learning and bilingualism in antiquity is a hot area of research at the moment!
Latin was mainly used to administer the Western Empire; Greek the Eastern Empire. Educated Romans could speak both throughout the entirety of the Empire, except that Greek began to decline in the West later on (Symmachus Ep. 4.20).
long-standing contact with peoples like the franks and of course the persians will have given them ample opportunity to learn the language, but how did they broker deals with the huns, who they had almost no direct contact with before
Actually, Romans very rarely learnt anything except Greek dialects and Latin. (Ovid, forced to learn Getic in the 1st C AD, is a famous exception). They expected peoples in the empire to learn the relevant language(s) rather than the other way around. When it comes to people outside the Empire, there were always ready interpreters at hand (the army was highly multilingual; every soldier knew Latin but many other languages were represented, so military leaders would have been able to call on these soldiers to interpret). Romans were accustomed to using interpreters in other contexts in any event.