In my household we have a curious situation: two of us are learning Arabic, but so far we cannot exchange much more than a ritual "Good morning." My 6-year-old son comes home from school mum on the subject, then demonstrates a startling facility for writing its fluid script, sweeping right to left across the page. Meanwhile, my colloquial Arabic class is drilling me with Useful Phrases, but I cannot so much as distinguish the letters on a stop sign. In short, we are living the famous diglossia of Arabic, where there is a spoken dialect that is rarely written and a written language that does not sound natural when spoken. And we’re wading through these currents in Lebanon, where English and French have strong competing presences, and rampant code-switching requires a knowledge of covert social rules. "Don’t greet someone with Salaam aleikum unless you know they’re very religious," one acquaintance advised. Use marhaba and shukran in this part of the city, bonjour and merci in that. Welcome (as in "You’re welcome,") and sorry (for "Pardon me") are part of the lingua franca–just add an accent and roll those r’s.

Lebanese Arabic would not get me very far outside the country, so perhaps it should not have come as such a surprise that English could get me so far within it. When parents aren’t code-switching with their children ("Don’t touch that!" snapped a mother in full abaya to her toddler at the candy counter, between admonishments in Arabic), they are speaking English in perfect imitation of a Hollywood movie whose title is forever on the tip of the tongue. "You have to make a choice," I overheard a father say to his son outside our children’s school. "Life is about making choices."

Including, one might add, choices about language. Am I immersed in the confluences of a vibrant bi- and tri-lingualism, or am I hearing what a dying language looks like? What would Claude Hagège say? Hagège is the chair of linguistic theory at the College de France, and his new book, On the Death and Life of Languages (Yale; $30), translated by Jody Gladding, is a passionate defense of diversity in the face of language extinction. A dead language is one with no native speakers; Hagège measures the health of a living language by how vigorously it is handed down from parent to child. "The fact that a language stops being transmitted to children as it usually is under natural living conditions is the indication of a significant jeopardization." Since 2,500 of the world’s current stock of 5,000 languages are predicted to die by the end of the century, it is not unimaginable that Lebanese Arabic could be among them. (A recent UN report estimates the number of existing languages at closer to 7,000, of which 90 percent of the languages spoken by indigenous peoples should be obliterated at an even faster rate.) While anxious that their children learn written Arabic at school, it is clear that many of the highly educated parents I meet speak English at home, at least some of the time. Does that make them bilingual cosmopolitans, or "under-users" of Arabic? Are they merely employing "lexical borrowing" (using foreign vocabulary that does not have a native counterpart), or is the dialect in the beginning stages of erosion?

Hagège distinguishes between the sort of bilingualism we claim to value–one of benevolent diversity–and "inegalitarian bilingualism," the gradual takeover by one language of another. When the inegalitarian variety is a consequence of economic advantage, the language with less prestige goes under fast. Code-switching hardens into substitution, and vocabulary is lost. Eventually structures–morphemes, morphology–follow. A society withers away. Call it natural selection, or cultural climate change.

The singularity of a culture, however, is not just an expendable bit of enrichment, like the elective French you can drop for econ if you have to. What is lost, says Hagège, is a world. Languages are "reflections of the infinite" in at least two ways: they preserve the past for us–something no other animal species has the means to do–and they conjure the hypothetical, a function crucial to poetry and fiction as well as the sciences, philosophy and law. Lose a language, and you lose your history and your visionary capacity.

These are the macro losses; on the micro level, all kinds of experience, from natural phenomena to domestic habits, are wiped from memory. Hagège rues the diminution of Kusaiean, the Micronesian language spoken on the easternmost island of the Caroline archipelago, which no longer employs twenty-eight terms for the moon’s phases. Kamilaroi, an Australian language spoken in New South Wales, "has lost nearly all the fine distinctions its verbal system made between the different times of the day that, from sunrise to sunset, served to frame events." In perhaps the most dreadful image in the book, Hagège evokes elderly immigrants so displaced that they have forgotten their language and have been rendered mute. He writes about "Armenians of advanced age in Marseilles or other French cities who are no longer capable of coherent discourse in Armenian and babble almost incomprehensible syllables." Muteness occurs by degrees, but the end result is that limit and measure are withdrawn from both the heavenly bodies and the personal body, leaving the self-marooned in primordial chaos.

If language loss is a collective phenomenon ("A language in good health is willingly valued by its speakers"), can there be such thing as collective language guilt? I wondered this while reading Hagège’s account of Hungarian–an agrarian language that has in places been swamped by the more technologically modern German–and stumbling across a black and white photograph, on the Internet, of two kerchiefed Hungarian tots and their mother praying at a grave on the Day of the Dead a century ago. I grew up hearing my father speak Hungarian to his parents without ever learning it myself, and now I asked myself irrationally what I was doing learning Arabic when I didn’t know either my father’s or my mother’s language (Byelorussian) very well. Are my language experiences compromised, doomed to be short-lived because they are rootless? Hagège: "A word cut off from its group of affinities by historical circumstances is defenselessly exposed to pressures that alter its semantics. That is because it is no longer integrated into a context capable of preserving it." If the languages I grew up hearing are endangered, I have contributed my share of indifference toward them. Do I feel guilty? Yes, a little. It’s not for nothing that even the most lyrical poets, revenants of their tribe’s old role as tribal record-keepers, pay close attention to etymologies.

My father never felt guilty for not teaching his kids Hungarian. The Yaaku, the north-central Kenyan people who voluntarily renounced their language in the 1930s when they abandoned nomadism and joined the livestock-keeping Masai, probably did not feel guilty. I can’t speak for the parents around me who teach English to their children, but here in Beirut, the capital of code-switching and diglossia, the fracture of language can be as exhilarating as it is equivocal. Who knows where it’s headed?

For all his attention to the death of languages, Hagège is not an undertaker. He recounts the story of the revitalization of Hebrew after its millennia-long slumber as a liturgical language; he points out that, historically, many tongues survived their conquerors. The Norman conquest in 1066 instituted French as the prestige language of the English court; by the end of the thirteenth century a vigorous Latinized English was Chaucer’s medium, but French had dwindled away. Likewise, Persian and Turkish have survived Arabic domination, and Japanese, Korean and Vietnamese have survived Chinese. Current, ongoing renaissances of dead languages include Cornish, Welsh, Breton and Scottish Gaelic; Yiddish; Neo-Norwegian, or Nynorsk; and many others. A pragmatist would roll his eyes. Language is for communication, after all. The paradox is that we need language to communicate, but if language were just for communication, it wouldn’t matter so much to us. It wouldn’t make us feel guilty. It wouldn’t inspire love.