Like how would Marco Polo figured out how to speak with the Mongols? Was it like 20 translators each communicating the message to each other? In the Americas was it a trial and error process? Were explorers ever able to fluently speak their new languages if they acquired them?
I can help with the Mongol part at least, here are some similar previous questions I've worked on:
How powerful were translators during ancient, medieval or early modern times?
How would someone learn a foreign language in High Middle Ages?