A language dies when there is nobody left to speak it.
By the best estimates, around 6,000 languages are alive in the world today. Half of them, perhaps more, will die in the next century — that's 1,200 months from now. So this means that somewhere in the world, a language dies about every two weeks.
Why do languages die? There are many reasons — natural disasters (for instance, if an entire village of speakers is killed in a flood, or wiped out in a disease epidemic), social assimilation (speakers cease using their native language and adopt a more popular language in response to economic, cultural, or political pressures). Genocide, colonization, and forced language extinction are causes.
The belief that language diversity is healthy and necessary is often compared to biodiversity, and the idea that a wide array of living species is essential to the planet's well-being.
Michael E. Krauss, of the University of Alaska's Alaska Native Language Center, extends this analogy to define three stages of language health in The World's Languages in Crisis:
moribund: "languages no longer being learned as mother-tongue by children"
endangered: "languages which, though now still being learned by children, will — if the present conditions continue — cease to be learned by children during the coming century," and
safe: languages with 'official state support and very large numbers of speakers.'
If we measure the value of a language simply by the number of people it allows us to communicate with, bigger would always be better, and the death of an endangered language would be of no consequence to the rest of the world. If 128 million people speak French, and roughly 100 people speak Pomo — a nearly extinct indigenous language in California — then French is exponentially more valuable than Pomo.
But language is not math. Language is embodiment of cultural identity. Language is nuance, context, place, history, ancestry. Language is an animate being; it evolves, it adapts, it grows. Language is the unique, neural fingerprint of a people. Language is a living code that provides structure for human experience. Language is intellectual DNA.