For example, in wars, how would the kings communicate with each other? There might have been some people who knew multiple languages, hence served as translators, but what if a civilization got into contact with a new civilization, who’s language no one knew?
Ancient and medieval people pretty much never encountered a completely new civilization with a totally foreign language. There were always interpreters and translators, and common languages spoken by different cultures (Greek, Latin, Aramaic, Persian, etc).
We have a section in the FAQ about this, which will hopefully be helpful:
Medieval monarchs tended to speak multiple languages themselves. Whilst he reigned slightly after the medieval period, Henry VIII is a good example as he spoke French, Latin and Spanish. Even if medieval kings did not speak each other’s languages, all European kings would have been able to speak Latin as it was the language of scholars at the time, with most books and official documents being written in Latin.