John McWhorter devotes one of the chapters of The Power of Babel to language extinction.
“[I]n the past languages have usually gone extinct when one group conquers another or when a group opts for a language that it perceives as affording it greater access to resources it perceives as necessary to survival. Typically, a generation of speakers of a language becomes bilingual in one spoken by a group that is politically dominant or endowed with valuable goods or access to same. … Usually, through time new generations come to associate the outside language with status and upward mobility and indigenous one with ‘backwardness.’ …
“A point arrives when one generation speaks the outside language better than the indigenous language, largely using the latter to speak with older relatives and in ritual functions. As such, these people do not speak the indigenous language much better than many Americans might speak French or Spanish after a few years of lessons in high school. One is unlikely to speak to one’s child in a language one is not fully comfortable in and does not consider an expression of oneself. It is here that a language dies, because a language can only be passed on intact as a mother tongue to children.”
But a language can be saved if it is written down, can’t it? No, says McWhorter. “Most of us can attest to this from our exposure to Latin – no matter how good you may have gotten at those declensions, conjugations, and ablative absolutes, even this was a long way from speaking the language fluently … Languages die when others take their place – we don’t need Latin or any dead language, because we’ve got languages of our own. As often as not, a revived language hovers in the realm of the ‘undead’ – part of the revivification effort entails gamely making space for the language in lives already quite full without it and sometimes even vaguely discomfitted by its return.”