Everyone knows that it’s moving and melancholic to watch your children change over the years. But to hear them alter their language, over the course of a few weeks and months, is almost surreal. It’s as if the precious beings you thought you knew are completely different and the experience is both intriguing and unsettling.

Our children were 12, 10 and seven when we moved from Somerset to their mother’s country, Italy, last summer. Until then, they had always lived in England and their English was what you would expect: the odd spelling mistake, but otherwise fluent and full of pre-teen playground slang.

Now, in Parma, barely a day goes by when they don’t inadvertently say something odd: “Mum, I’m eshing [going out]”; “Can we eat pesh? [fish]”; “I’ve scritten [written] to Grandpa”; “Can you accorten [shorten] my trousers?”; “Have you chiused [closed] the door?”; “Shut up, I’m parling [talking]”; “I’ve strapped [ripped] the page”. Every time it happens we laugh about our private pidgin, called – take your pick – “Engaliano” or “Italish”. But behind the laughter is mild astonishment at the speed at which children can overlay and overlap languages.

Sometimes, they’re not inventing words, but using the English ones in an Italian way: “I have been advantaged,” Benedetta said a few weeks ago, meaning: “I’ve been fortunate”. “I lost the bus,” Emma said, implying, of course, “I missed it”. “She’s an insupportable person” is an Italish way of saying: “I can’t put up with her”.

Their pronunciation is changing, too: Leo no longer says “flash” (his televisual obsession) but, like his schoolmates, “flesh”; he doesn’t talk about “plum cakes”, that mass-produced Italian snack, but “plume cegs”. All their hand gestures are Italian (bouncing the fingertips, all together, towards the chest to say: “Dad, you’re talking nonsense”), and, inevitably, they’re trying out Italian swearwords. Until we moved here, they were what would be called “receptive bilinguals”: they understood everything in Italian but almost never spoke it. Because my wife, Francesca, is a self-confessed assimilator – never wanting to appear foreign in England – she would invariably speak to them in immaculate English in public.

Because we were living in a community, having set up a woodland refuge in Somerset, we were always in public. Only when they were very young did our kids speak Italian as easily as English. I remember taking two-year old Benedetta to a Six Nations rugby match. She was shouting “dai, dai” to the Italian players, meaning “come on, come on”. All the Welsh supporters thought the sweet girl in pigtails was shouting “die, die”.

Facebook Twitter Pinterest The children are already equally comfortable with reading Italian and English. Photograph: Andy Hall/The Observer

The last few months have been like that blissful period when your baby starts making intelligible sounds and words for the first time. You’re constantly shocked by their skill, by their humour and misunderstandings. Only this time, the beings surprising you, and themselves, are no longer babies, but much older.

What’s breathtaking isn’t just their language acquisition, it’s the way their personalities subtly mutate and shift. Benedetta, a bruiser at the best of times, is strangely sweet and gentle in Italian; Emma, our incessant court jester in English, is precise and serious; and Leo, who has always seemed stereotypically Italian, is even more boisterous. It feels as if our children are different, more mature, but also, because of linguistic struggles, somehow more infantile or vulnerable.

I, too, feel altered. If you’re a writer, and words are your currency, you’re hopefully eloquent, maybe even wise. But when you live in a language not your own you become, inevitably, something of a village idiot. However fluent you are, your accent is ever obvious and you lack the nuances and registers of a native. Humour is hard. In a language as mellifluous as Italian, you will always be blunt, a bit like a German speaking English.

You can turn such things to your advantage, of course: Italian is often spoken for the sound rather than the meaning, for the cadences and colours and flourishes… so a foreigner’s bluntness is often, Italian friends generously say, a relief. Even being a village idiot can be turned to your journalistic advantage: for years, while on investigative pieces in Italy, I’ve used what I call the “Columbo technique”, deliberately appearing more dippy and bungling than I am to catch people off-guard.

So if the children appear differently to us, so must we to them. From having been a stickler for linguistic precision, their father must seem an erratic guide: they can hear my mispronunciations and absent subjunctives. After such a short period, their mother’s faultless English has slipped a notch or two. They’re realising that they’re due to overtake both of us in a matter of months. They will hopefully become what Francesca and I can never be: truly bilingual.

Until recent decades, bilingualism was deeply frowned upon and considered deleterious to development. The received wisdom for much of the 20th century was that there was really only space for one language in a child’s brain. It was thought that if, for example, immigrants maintained a mother tongue at home, it would impede integration at school and probably lead to academic regression and confusion.

Facebook Twitter Pinterest In Los Angeles, 60% of the population is functionally bilingual. Photograph: Soltan Frédéric/Getty Images

As one journal study put it in 1926: “The use of a foreign language in the home is one of the chief factors in producing mental retardation as measured by intelligence tests.” The choice to avoid the mother tongue wasn’t simply an educational, but also a social, one; the home language was invariably considered a source of shame, a sign of poverty or difference that would almost certainly lead to being singled out and bullied. For immigrants, integration used to imply the deliberate avoidance of your parents’ language, at least in any public setting. The pendulum began to swing back, very slowly, after 1962, when an academic study was conducted into monoglot French speakers and English-French bilinguals in Montreal. The authors, Elizabeth Peal and Wallace E Lambert, had expected a variety of tests to prove that the monoglots were more able than the bilinguals, but the exact opposite was the case.

“Intellectually,” they said, “[the bilingual child’s] experience with two language systems seems to have left him [or her] with a mental flexibility, a superiority in concept formation, a more diversified set of mental abilities.” That dry prose of the monograph, The Relation of Bilingualism to Intelligence, disguised a revolution: for the first time, academics were suggesting that, far from being a hindrance, exposure to more than one language could offer a distinct advantage.

Over the next two decades, various academics kept pushing that pendulum, devising aptitude tests that revealed that bilinguals were marginally more competent at problem solving, meta-linguistic awareness and symbol manipulation. The more tests were done, the more neurologists and cognitive scientists focused on “executive control”, the cognitive skills in the brain’s frontal lobes that support high-level thought, memory, attention and multitasking.

The effects of bilingualism on neuroplasticity (the modification of brain structure and function) seemed clear cut, so researchers needed to understand how the brain accessed the two (or more) languages. What emerged, through hundreds of experiments, was the notion that rather than having two different “pockets” containing, say, French and English, the bilingual’s brain had one huge holdall for both. (Not everyone agrees: linguists are more likely than psychologists to believe that languages are separate, the grammars even more so than the vocabularies.) Either way, in any linguistic setting, bilinguals’ brains would begin “joint activation”, rummaging to find the right words in the appropriate language. That requires, of course, immediate inhibition and activation, stopping the wrong language and authorising the right one. The result is what’s called “enhanced attentional control”, with obvious implications for concentration, memory and so on.

The effects of bilingualism appear to take place almost from birth. Extraordinary experiments – involving electric sensors in babies’ dummies – suggested that infants only a few months old can distinguish one language from the next. Eight-month-old infants are able to distinguish the facial expressions of one language from another. If such “enhanced perceptual attentiveness” is evident so early on, it’s perhaps not surprising that the bilingual brain seems to be wired differently. In another experiment, eight-year-old bilingual girls displayed enhanced “complex spatial reasoning” compared to their monoglot peers; less subject to “egocentric error”, they were much more adept at understanding what the arrangement of four coloured blocks would look like from 90, 180 and 270 degrees.

Along with a host of other research, implied in the results is an ability to empathise, to see a situation from another’s perspective. It is, perhaps, obvious that if you’re habitually changing your language to accommodate the interlocutor and their context, you’re inevitably going to be used to taking into account other people’s abilities and points of view. It’s called, in the technical jargon, “code-switching”, the most famous example of which came from Charles V, the pan-European emperor of the 16th century: “I speak Spanish to God, French to men, Italian to women and German to my horse.”

At the other end of the life spectrum, there appear to be positive consequences of bilingualism. One of the foremost researchers into the subject, the Canadian psychologist Ellen Bialystok, analysed the medical records of patients diagnosed with dementia and discovered that onset symptoms were diagnosed between three and four years later in bilinguals (on average at 78.6 years old, compared to 75.4 years for monoglots).

No one is implying that if you’re bilingual you won’t get Alzheimer’s. But the research does suggest that bilingualism, like other mentally challenging activities such as playing the piano or chess, can attenuate age-related deterioration. The orthodoxy is the exact opposite of what it was 50 years ago, with flag-waving books called The Bilingual Edge and The Bilingual Advantage being published. It would be very easy to become smug, but there are clear drawbacks as well as advantages. The verbal aptitude of bilinguals in each language is generally less than that of monolingual speakers; the vocabulary size is smaller and there is a slower response time among bilinguals for the understanding and producing of words.

Facebook Twitter Pinterest French president Emmanuel Macron, with his wife Brigitte, wants to make French a global language. Photograph: Witt/SIPA/REX/Shutterstock

It’s as if some bilinguals can speak, say, 95% of two languages, rather than a 100% of one. For those who want their children to be articulate and sophisticated language users, that missing 5% is a major hole, especially if you want them fully to appreciate, say, Dante or Shakespeare. But as François Grosjean, the French psycholinguistic said, a bilingual is not “two monolinguals in one” and shouldn’t necessarily be judged according to monolingual parameters.

In November last year, there was a good news story that almost sank without trace. It wasn’t just uplifting because St Stephens in East Ham, London, was the first non-independent school to top the Sunday Times school league table. What made it remarkable was the fact that 96% of students were EALs (speaking “English as an additional language”). Here, it seemed, was proof of what academic research had been saying for years: that the maintenance of a “home language” may be beneficial for learning the “community language”, that proficiency in that first tongue enables proficiency in the next. Antonella Sorace is the director of Bilingualism Matters, a research and information centre at the University of Edinburgh that promotes multilingualism in Scotland and across the globe. “For decades,” she says, “there was this notion that learning two languages together, or too soon, would affect children. It would cause problems at school. That’s always the message that parents got, but there’s absolutely no evidence of that; in fact, quite the opposite.”

The research that finally put that prejudice to bed was conducted by Jim Cummins and Virginia Collier at the University of Toronto. They discovered that students who had a native language literacy would display CALP (“cognitive academic language proficiency”) in a secondary language far sooner than those who didn’t have a strong native language. The implication was that it’s far more advantageous for immigrant children to hear an eloquent, grammatically correct, richly nuanced language at home than be exposed to low-level pidgin English. They can then transfer those language skills – the concepts, diction and sophisticated structures – to their new language.

It shouldn’t come as a surprise that so much cutting-edge research into bilingualism, from Cummins and Collier to Bialystok, comes from Toronto. It’s a city in which it is estimated that half the inhabitants are bilingual. But those figures are now the norm: 56% of European citizens and 60% of Los Angeles residents are functionally bilingual. Globally, the figure for bi- and multilingualism is thought to be over 50% and rising, meaning that being monolingual is no longer (if it ever was) the default to which polyglots are an odd exception.

But there are sociological, as well as demographic, reasons why bilingualism is no longer frowned upon. In the early 21st century, there is far less certainty that a developed or colonial country’s language is somehow superior or nobler than another. Thanks to multiculturalism, immigrants have less of an inferiority complex and “natives” are, mostly, more accepting of diversity, linguistic as well as racial or sexual. More subtly, we’re aware that there are endangered languages as well as endangered species: the organisation Ethnologue has suggested that 473 tongues are close to extinction. Hence the existence of the Linguapax prize, awarded on International Mother Tongue Day (21 February) to celebrate work that treasures linguistic diversity. Perhaps most importantly, international communication and movement are so simple that the maintenance of the mother tongue isn’t only far easier, but is also – in an age of globalisation – economically advantageous. Studies have shown that bilingualism has boosted Switzerland’s GDP by 10% and that, on average, bilinguals in America earn $3,000 more per annum than their monolingual counterparts. But perhaps bilingualism is also a response to a world in which nationalists, protectionists and isolationists seem to be goose-stepping again. Language has always been a weapon of nationalists: in the Bible, in Judges 12, Ephraim is killed for mispronouncing the word “shibboleth” and revealing himself an alien to the Gileadites. It’s impossible to judge cause and effect, but the fact that the excellent language learning app, Duolingo, has more than a million registered Esperanto users, and that Wikipedia has a quarter of a million articles written in Esperanto, must say something about peoples’ yearning to build bridges, not walls. Esperanto means “one who hopes” and the spiritual side of that constructed language was the promotion of world peace.

It may seem that the battle is won and that no one nowadays would question the virtues of bilingualism. Yet supporters of multilingualism, such as Antonella Sorace, insist there is much work to do. It’s very common to hear people tut-tutting at immigrants using their own language in the playground, because they believe it’s a block to integration. There’s plenty of anecdotal evidence that parents are being told by schools to stop speaking their own language at home. Plenty of nationalists still think that “code-switching” sounds traitorous, a bit like being a double agent, hence the existence of reactionary lobbies such as the “English-only movement” in the US and its equivalents for Hindi in India. Emmanuel Macron’s ambition, expressed last month, for French to become the global lingua franca shows how hard it is to let linguistic pride go.

In recent years, certain separatist movements have assiduously promoted their own tongue

In recent years, certain separatist movements have assiduously promoted their own tongue and, inadvertently or deliberately, reduced the use of another. In 1977, the Charter of the French Language made French the official language in Quebec, leading to a drastic reduction in native English speakers (down by 180,000 according to the 2006 census). When an independence movement becomes identified with linguistic assertion, as in the Basque country, Catalonia or Wales, bilingualism can be seen as an affront to the aspiring nation struggling against the “colonialist power”. The result is that, for some, the use of Castilian or English is seen as a betrayal. It’s like seeing Brian Friel’s brilliant play about the erosion of language, Translations, played backwards, the erosion now taking place the other way.

There’s an old controversial theory called the Sapir-Whorf Hypothesis. Growing from the work of American anthropologist Franz Boas (the man who famously noticed that Inuit had multiple words for snow), the hypothesis suggests that the language you speak affects the way you conceptualise your surroundings. The fact that, for example, the Amondawa people of the Amazon have no word for “time” or that neither Gaelic nor Latin has precise equivalents for “yes” or “no”, surely changes how those speakers make their way in the world.

Even the mental images we have for the same notions are different: icing sugar in Italian is “veil sugar”, while in French you know someone not like “the back of your hand”, but like “the bottom of your pocket”. I’m frequently frustrated that I can’t find exact equivalents of terms I need to deploy in Italian, such as “condescending” or “accountability”. Italian words don’t fit what I mean. It’s intriguing, too, that “reality” in Italian is sometimes plural, as if there’s no solid truth we can get hold of here, but just competing versions of it.

It’s easy to jump to silly conclusions, which is why the Sapir-Whorf Hypothesis is controversial. It’s so prone to reductionisms: “If the English have those linguistic voids, lacking a way to say ‘simpatico’ or ‘buon appetito’, they must be boorish anti-foodies.” Without equivalents of “accountability”, the lazy thinking goes, Italians must be slippery or unreliable. Yet, for all its controversies, the hypothesis – known also as “linguistic relativism” – surely points to a truth: that the more languages we know, the more agile our conceptual thinking will be.

In another language, you don’t just learn new words, or sounds, but new notions. It’s like putting on different spectacles and seeing the world with different eyes. You gain a different perspective and sometimes, if you’re lucky, you become more, rather than less, eloquent.

Tobias Jones lives in Parma. He’s writing a book on Italian Ultras. www.tobias-jones.co.uk