When I was a boy London was a dowdy place of tea-houses and stale rock cakes where everybody spoke English. When I come back now it's much more exciting – and not just because the food has improved. On the 37 bus, on Kilburn High Road and in Sainsbury's, I can hear people speaking in all the languages of the world. Was that Pashto or Hindi I heard in the queue? I can just about differentiate Polish from Lithuanian, and delight in hearing them mingled with snatches of French, German, Spanish, Italian, Japanese … London has become the capital of linguistic diversity.

But there's one important group that seems to be leaving itself out: students. Foreign language learning at Britain's schools has been in decline for decades, while the number of universities offering degrees in modern languages has plummeted. Indeed, an inquiry is now under way after the number of teenagers taking traditional modern foreign languages at A-level fell to its lowest level since the mid-90s. It's a paradox.

There's a widespread notion that since English is the new Latin, native speakers don't need to learn to speak anything else. The Romans were not so foolish. Their empire was populated almost exclusively by bilinguals. In a cemetery near Newcastle upon Tyne, there's a Roman gravestone inscribed in Latin and in Palmyrean (the ancient kingdom situated in what is now Syria). The eastern part of the Roman empire used Greek, not Latin, and most people around the eastern Mediterranean spoke Aramaic, Egyptian, Punic or one of a welter of other languages along with Greek or/and Latin. The idea that because English is now a widely spoken international language, English speakers need nothing else is then very much a modern invention, with no precedent in older civilisations.

Of course, there have also been monolingual societies in the past, but I'm not sure any of today's youngsters would like the kind of life that they offered. Edo Japan (from the 17th to the mid-19th century) allowed its people no contact at all with the outside world. But we live in the age of easyJet!

There was a time when everyone – meaning the European elite – spoke French. But that was a time when most French people were themselves bilingual, speaking a regional language (Provençal, Breton, Catalan or Alsatian). Today too, when "everyone speaks English", most of its speakers worldwide also speak something else. Just one small group (as a proportion of English speakers overall, it is very small indeed) seems set on isolating itself from the world: British schoolchildren who have no second language at home.

Language learning doesn't take the brains you need for maths – the evidence being the far wider distribution of bilinguals than competent mathematicians. But it does get harder with age, and the best time to acquire a second language is before you're 18 (the earlier the better, in fact). Like maths and music, languages sink in faster and deeper before concerns about sex and jobs besiege the pliant brain. Why in fact do our schools teach anything other than music, maths and foreign languages? These are foundational skills that make other knowledge much easier to acquire later on.

Many schools have a fantastic linguistic resource – their own students. Yet few of them are encouraged to learn Bengali, Urdu or Polish in the playground, and I'm not aware that any school has tried to foster or formalise peer group learning of that kind. What a waste.

Britain is not unique in turning away from languages at school. Only a small number of students in Germany acquire French; in France, outside border areas, the proportion taking Italian, Spanish or German at the baccalaureate exam is pitiful.

But our European neighbours also learn English in large numbers, and soon the only monoglots left will be British. A monolingual society in Britain is no more likely to be successful than those that exist in the Amazon jungle and New Guinea. The upper classes will presumably continue to cultivate languages because elites know how to reproduce themselves (the present cabinet is the most polyglot in recent history). But schools are failing everybody else by not insisting that a far larger proportion of the rising generation acquire a good knowledge of at least one foreign tongue.