We should be or become multilingual. It is not optional for being a well-rounded human being. The limits of language impose limits on our thought, and different languages impose different limits and open up different areas of thought more easily. No language opens up the complete range of human thought, and probably not even all human languages taken together would do so, but learning more than one language at least lets us expand our mental horizons. There are other arguments to be made for multilingualism, such as the utility and improved harmony of being able to communicate with more of the world's people, but you have heard all that before.
So, if we should learn more than one language, which ones should we learn?
First, we should master our own native language or languages, in my case English, since I'm such a mutt that I don't have a short list of ancestral family languages to add. Only death should be the end of that learning process: we can never be fluent enough to justify quitting. Even as we should push at the boundaries of our thinking with philosophy, so should we push at the boundaries of our fluency.
Second, we should learn the languages of large numbers of our immediate neighbors, so we can be good neighbors. In most of the world this is more of a challenge than where I live, but even in the United States, we have Mexico and Canada as neighbors, and numerous immigrants from around the world in the U.S. Wikipedia has good information on languages used in Mexico, Canada, and the United States, but I will offer a summary. For good neighborliness, any North American should be at least bilingual, speaking English and Spanish, each of which has over a hundred million speakers locally. It would be better for North Americans to speak French as well, since almost ten million of our neighbors do. Millions of our immediate neighbors speak Chinese (2.85), American Sign Language (between .5 and 2), German (1.84), Nahuatl (1.5), Maya (1.5), Italian (1.48), Tagalog (1.37), and Vietnamese (1.13). Hundreds of thousands of our immediate neighbors speak Korean (975), Polish (878), Arabic (810), Russian (804), Hindi & Urdu (716), Japanese (514), Greek (490), Persian (404), Gujarati (297), Punjabi (over 271), Armenian (227), Navajo (178), Ukrainian (over 148), and Dutch (over 128). I will be refreshing my French this year, trying to tune it up enough to read a massive French series on Heraclitus. I want to refresh my Spanish after that, next year or the year after, depending on how long it takes me to improve my French enough. After that, in the years ahead for neighborliness I want to pick up American Sign Language and Japanese (because of Seattle's special relationship with Japan).
Third, we should learn our religious and philosophical languages, including Arabic, Ancient Greek, Latin, Ancient Hebrew, and the other languages of the world's great ethical systems. I will begin with Ancient Greek this year to support my cultural and philosophical studies. Arabic will be next, but not for a while.
Fourth, we should learn the languages of the First Peoples where we live, in part to show respect, in part to learn the languages of people who lived where we live far longer than we have and whose languages may well be more fully and subtly adapted to our homeland than our "native" tongues. As a Seattlite, I need to learn Salishan, but it will be a few years before I begin.
Fifth, we should learn the major languages of the world. The Linguasphere website has a great article on the languages most spoken around the world. Two languages have a billion speakers: English and Mandarin (aka Putonghua). Two pairs of related languages have a half billion speakers: Hindi + Urdu, and Spanish + Portuguese. Three more languages have over 200 million speakers: Russian, Bengali, and Arabic. Four more have 100 million or more speakers: Japanese, Malay + Indonesian, German, and French. From among these I am initially interested in adding Mandarin and Hindi + Urdu.
Sixth, there are always the languages we pick up because of our own personal interests. Because of my history studies to improve my Dungeons and Dragons games, I have sporadically studied Anglo-Saxon. Because of my love of The Lord of the Rings, I used to haltingly speak and write Tolkien's Sindarin and Quenya. Because of idealism, I spent some time learning Esperanto. Because of its long history as a scholarly lingua franca in the three millennia BC like Latin later became, I am very curious about Sumerian.
Of all of these, I am only fluent in English, I can reasonably understand and make myself understood in Spanish (which I used to be fluent in enough even to understand Portuguese), and I can hobble along in French. For German, Italian, Japanese, Chinese, Anglo-Saxon, and Ancient Greek, I only have a few words and phrases and some practice with pronunciation. This is not nearly good enough. For at least the reasons outlined above, we all need to become polyglots, and it is not even that difficult. If we pick up a new language book and tapes every year or two and read and practice in our spare time over the course of the year, we can at least become fluent enough to understand and make ourselves be understood in the basics in a surprising number of languages.
It is a big world full of many interesting people. Let's make it easier to meet them by learning their languages, and by doing that work make ourselves more interesting to meet as well.