If you've been reading my blog and so far been unimpressed by my praise of learning languages besides one's native language, perhaps you will become intrigued when I tell you that learning a language is a lot like learning to play a musical instrument. There are a few lucky geniuses who can master many musical instruments or many languages with ease. For most people, however, certainly including me, studying music and studying languages involves a lot of excruciatingly hard work, especially at first. But if you persist long enough in studying music or a language, eventually you will achieve some measure of mastery, and achieving even a little of that is wonderful and thrilling in a way which is very difficult or impossible to explain to someone who hasn't. So if you can play an instrument, or several instruments, believe me: the rewards of studying a languages, or languages, are comparable.
The term "barbarian" was coined by some ancient Greek who didn't speak any languages other than Greek, wasn't interested in learning any more languages, and didn't listen very carefully to anyone speaking anything other than Greek, because the original definition of a barbarian was someone who didn't speak Greek, and there is no language that's all b's. When Rome conquered regions from present-day Greece to Egypt to Syria in the 2nd and 1st centuries BC, some of which had spoken Greek for over 1000 years and others of which had been conquered by Alexander, the Latin language didn't spread into the eastern part of Rome's dominions, as Latin had become dominant in all of the western regions; the Greeks who were the majority population in some of those areas and the ruling class in the rest kept right on speaking and writing in Greek, and a great number of Romans acquired Greek as a 2nd language. Politically, Rome had conquered Greece, but culturally, Greece conquered Rome.
Paradoxically, the Greeks acquired the disadvantage of monolingualism from this cultural dominance. From the 17th to the early 20th century, French culture dominated Europe, resulting in a similar paradoxical disadvantage for French people: many more English and American people spoke both English and French, many more Germans spoke both German and French, many more Russians spoke both Russian and French, etc, than the number of Frenchmen and -women who spoke French and anything else. And today, the US is the political and cultural leader of the world, resulting in the paradoxical disadvantage for the US and other English-speaking lands of a paucity of people who speak anything other than English, compared to the numbers of, for example, French people who speak both French and English, or the number of Mexicans who speak both Spanish and English, if not an indigenous language and Spanish and English. A century ago it was still considered quite wise for Americans traveling to most other parts of the world to learn at least a little French before they traveled, and ideally much more than a little. Today, not so much.
This paradoxical benefit of being conquered culturally is very real. But as I mentioned above, I don't know any way of telling you how great that benefit is if you don't learn other languages and see for yourself. In the meantime all I can think of to do is to urge you to believe me when I tell you that the benefit is immense.