Every two weeks, a language dies. A set of sounds, evolved over centuries to communicate love, grief and the best place to find water, simply peters out. Linguists squabble over the exact moment of extinction. Is it when the last speaker of a mother tongue dies? Or should that be the penultimate speaker (after all, if there's no one to talk to, then the language has already outlived its usefulness)? Is a language still viable if only a few 50-year- olds can speak it haltingly alongside their more habitual French, Spanish or English? And in any case, does it really matter? If people find that Welsh, Warungu or Cree no longer suits them, why worry if they want to give it up?

Andrew Dalby and John McWhorter are committed, in slightly different ways, to showing us how and why "tall building" languages are taking over the world, in the process silencing the mother tongues of communities from highland China to sea-level South America by way of the African interior. McWhorter, a Berkeley-based linguist, uses a broadly Darwinian model - hence the subtitle of his book - to explain the falling away of minority languages. Dalby, on the other hand, worries that this "survival of the fittest" approach implies that there is something intrinsically weak or faulty about a language that vanishes (many of these "primitive" tongues turn out to be infinitely richer and more complex than the bargain-basement English). Instead, he maps a process whereby an individual typically learns Spanish or German for its economic and social advantages, finds his native language less and less useful, and gradually ceases to speak it to his children and grandchildren. The distinction between a language killing itself and being lost is subtle but significant.

Dalby uses the spread of Latin under the Roman empire as a template for understanding how English has achieved its stranglehold over the world today: 400 million use it as a second language, 300 million as their first. The routes of transmission are historically the same: colonisation, government and trade. Like Latin, too, English has remained dominant long after the collapse of the political structures that delivered it to the four corners of the empire. There does remain, however, one important difference. In these times of ethnic jumpiness, English is increasingly useful, in a way that Latin never was, as a "vanilla" language that remains unaligned to any sectional interest. This is particularly so in Nigeria and India, where the special status allotted to Hausa and Hindi gives rise to grumbling among those who speak other mother tongues. English may be the language of the oppressor, but at least it oppresses everyone equally.

Dalby is particularly good at empathising with the mindset of a speaker of a minority language. Surprisingly, men are more likely to hang on to their native language or regional dialect, especially those who work in a traditional trade. It is women, with their need to socialise and their desire for something different for their children, who are more likely to move away from their mother tongue towards the dominant language. This impulse is fantastically strong, even in communities that have made a virtue of holding themselves aloof from their hosts and neighbours.

Linguists such as Dalby are caught in a double bind. On the one hand, they abhor those who fuss and fume about the way that standards of speech are "slipping". They remind anyone who will listen that language is a living thing, and that to try to freeze it at a particular moment - in the case of English, say, somewhere around 1956 - is not only offensive, but also hopeless. And yet, there's no disguising that Dalby cannot bear the idea of allowing minority languages to disappear. Trying hard not to come over all romantic, like some latter-day Herder, he argues that when a language is lost, then so is a whole way of looking at the world. If English is deprived of the chance to scavenge from other languages, then it will lose its capacity to express new ideas. In two or three hundred years, it will reign supreme, but it will also have started to creak and rust.

McWhorter covers much the same ground as Dalby, but in a linguistically more self-conscious way. Terrified of seeming too stiff, he delivers his text in folksy Californian, complete with rocking-chair anecdotes drawn from his childhood. For a British-English speaker, this gives extra bite to meditations on how we are all increasingly bilingual, able to understand American English even if we can't reproduce it (although it has to be said that, on occasions, McWhorter is unable to return the compliment, such as when he tells us that Welsh is still spoken in England).

Describing the process of language loss is one thing. Deciding what to do about it is quite another. McWhorter understands how unrealistic it is to expect people to throw themselves into relearning the languages of their ancestors when they could just as easily be picking up German or Spanish. (Indeed, the only instance where this has happened has been in Israel, where, over the past 50 years, Hebrew has been revived from textual script to living, breathing language.) Dalby, too, believes that "stable bilingualism" is probably the best we can hope for, a situation where minority languages continue to be spoken, albeit falteringly, alongside all-conquering English.

Both these books can be read in two ways: first, as a fluent synthesis of fiendishly difficult theoretical and empirical material that has been drawn from 50 years of academic research; or second, as a lucky dip of interesting facts. Thus you will be fascinated to learn that the Puliyanese of south India barely talk at all after the age of 40. French used to be Latin. In the 15th century, someone from Kent would have problems fully understanding someone from London. Of all the words in the Oxford English Dictionary, 99 per cent are taken from other languages. The Fula language of West Africa has 16 genders. Creole is a type of language, not the name of one. In Cantonese, the word for "worry" is the same as the word for "paint", "thin", "oil", "swim", "friend" and "again" (it all depends on how you say it). Deaf people have an accent when they sign in a language that is not their own. The ancient Gauls spoke entirely in riddles. "Phoney", "gammy" and "roger" (as in "to copulate") all come from Romani. There are 1,652 mother tongues in India. Armed with this information, you will be able to amaze your friends or, if you choose the right tone a la Cantonese, bore your enemies to distraction.

Kathryn Hughes is a biographer and critic