Disclaimer: I know this question doesn't refer to a specific time or place. However the situation that I present is largely consistent across historical and geographic bounds, unless the answers prove me wrong.

I studied French all throughout high school, probably 4 hours a week of classtime and around that much in homework. I could google any word or grammatical concept I wanted to and instantly have a complete explanation, I had all sorts of flashcards and textbooks and grammar exercises and videos from native speakers, etc.

And yet after four years of high school French, my French was still terrible. I couldn't understand native speakers whether in person or in movies, and when I went to France for holiday one time, the people had no idea what I was saying.

So how were ancient people able to learn multiple foreign languages and effectively communicate in them? Not only did they not have resources like textbooks flashcards google etc, but peasants and lower class people would probably have barely any time to study due to work. Not to mention language teachers were probably very rare. The probability of them coming across native speakers was likely very low for most.

Even if someone did manage to perfectly learn a language's grammar and vocabulary in their home territory with textbooks or teachers, how intelligible were they to native speakers? I mentioned how even though I had been listening to recordings and videos of French native speakers for years, I was still basically intelligible to them when I visited. To give a less general example, I have met many immigrants to my country (US) who have lived here for years and have such thick accents that they are basically unintelligible.

To give some particularly interesting (to me) examples:

• How were Norsemen living in isolated Scandinavia able to learn Byzantine Greek - to such an effective degree where there are records of Norsmen traveling to Constantinople and being well received by the Byzantine King? Of those who then rode across the Middle East to trade at Baghdad, how were they able to learn Arabic?
• How were Muslims from east Africa (Mansa Musa) or Southeast Asia (Malaysia, Indonesia) able to travel all the way to Mecca for pilgrimage? Speaking Arabic, and supposedly many of the languages of the territories in between?
• How was Marco Polo able to communicate so well with Kublai Khan that he was appointed as the Khan's personal emissary? (It has been said that Marco Polo knew several languages besides Italian even before he arrived in China - I presume these included Persian and Arabic, but even still I doubt that these were spoken in the Khan's court in China).
• Jesus is said to have spoken Greek in addition to his native Aramaic. How and why would the son of a carpenter in Roman Judea have learned Greek? Could he speak Latin too? How frequently did Greeks travel to Jewish towns?