The most efficient way to learn a language is already known; I systematised it here, a post about a multilingual who learned to speak multiple languages as an adult for his opera career. (Also backed by computational linguistics.)
Learning languages when an adult is arduous, and it’s that which makes him impressive. Children, on the other hand, naturally acquire language and grammar where adults have to learn and unlearn various norms, such as word order or suffix endings, and where to apply them. Founder of cognitive linguistics Noam Chomsky said we should not speak of children learning languages, so much as growing them.
Somehow education in the UK, has it that everyone is allowed to learn languages by the time it is harder and they have less incentive to learn, around 12-13 rather than before 6 or even 12. That is stupid.
There are a host of cognitive benefits to speaking multiple languages, namely enhanced executive control. Executive control is the ability to allocate attention to one activity. For example, German conversation over the activity of Danish conversation (another Germanic language) exercises the brain. Contrary to being an inefficient use of time, languages provide the most genuinely transferable skills in trained attention and the ability to communicate openly and differently.
The ability to focus on any task and blinker out the irrelevant is requisite for success; no matter what the task is. Indeed, were this advice about younger language learning followed it would have a positive affect across each school. The pattern making and recognition of language is not parochial, but universal to understanding anything human. (Consider music or coding or dance, which are forms of language. The more social the language, the more fun to learn.) Language too isn’t merely a vessel for thought. True, there are thoughts separate from signs but in the social sphere such signs are what counts as thought.
Ideology and history can give the appearance that languages (like peoples) are more different than they are. Mandarin, for example, shares the subject-verb-object word order of English. Swedish, typically associated as unromantic and logical for being a Germanic language, has a beautiful sing-song rhythm reminiscent of Italian. And yet more division among language families where history and politics divide languages more than logic. Italian and Spanish, for instance, are so closely related that cognitive linguists consider them latin dialects—70% mutually intelligible is the benchmark. If you learn one you have open access to the other.
So how many languages should a human know? I read Jared Diamond’s What Traditional Societies Can Teach Us and some historical research to find out.
Europe is deemed a cosmopolitan mix of language, with citizens knowing 3 languages from a young age. Yet Europe west of Moscow has as few as 110 survived languages; the tiny Pacific island archipelago of Vanuatu has 138. Empires are typically homogenic which leads to the decline of minority languages: Latin to Spanish, Italian, Romanian, languages, English to pidgin, creoles and Spanglish; and compulsory Mandarin in vast China. On the one hand a single language makes communication easier, and on the other a single language limits the possibilities.
In traditional societies with many languages trading, the mean average of languages anyone has is five. The language used instinctively reflects the subject-matter, and the in-group versus out-group behaviour of society members. (Some languages exist with as few as 50 speakers, and that is remarkably the number required to begin a new language.)
The notion, then, that there is a threshold limit to learning languages is a disproven and xenophobic belief. Some linguists, like Alexander Arguelles, can speak as many as fifty. Historically also, humans knew more languages than today. Latin & Greek in the seventeenth century and then German and French in the eighteenth, were compulsory for everyone educated. In societies that interact and vary in scale from hunter gatherer bands, to tribes, to chiefdoms in geographic areas well-suited to diversity—like Papua New Guinea—it is normal to know three to fifteen languages.
In fact, it is politically homogenising countries, like the UK and France in the 21st century, which are the exception to the historical norm, just as monolinguals and even bilinguals are exceptions to the multingial rule. How much the NHS could save with people 2-5 years less susceptible to dementia! How much pleasure and edification children miss out on because of mere schooling policy – is baffling. How much politics is given a parochial in-group and we fail to embrace change. All of these could be aided through languages.