A couple of things:
1. Monolingual societies are relatively rare, and even so are more common now than they used to be. For a lot of history, certainly in Europe, if you could only speak one language you were probably an impoverished serf in some isolated village somewhere. Nobles, educated people, people who lived anywhere near borders, and traders were typically multi lingual. IOW between each end of a trading route you have a lot of people who can bridge the language gap.
2. A lot of the time trade was done by traders rather than the government. The Silk Road is a great example of this – the Roman and Chinese empires had a long and thriving trade route, but AFAIK they never had any actual contact, not even diplomats or emissaries. Each side just knew there was a large empire in a distant land at the other end of the trade route that had a large appetite for the luxury goods each produced. The link between the two was a series of trading routes, including links consisting of groups from neither empire.
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