Daniel Everett is best known for his controversial research into the Pirahã language, which he popularised in a book called Don’t Sleep, There Are Snakes (steadily crawling up my to-read mountain.) The post title is a phrase adapted from Carlos Fuentes, which Everett used in a talk titled “Endangered Languages and Lost Knowledge”:
[T]he general principle that makes languages alike or different is very simple. You talk like who you talk with, so if you talk with somebody all the time, you’ll talk like them, and if you don’t talk to them, eventually you won’t talk like them at all. So, languages live like bread and love, by being shared with others.
But languages die also, and languages die in one of two ways. First way is that the speakers actually die, and so if the speakers of a language die out the language is going to die . . . . Another reason languages die is because the speakers stop speaking – speakers lived but they shifted to another language. So, the languages that are gone, usually won’t come back.
The full lecture, delivered at the Long Now Foundation, is on Fora.tv, where you can download the video, audio, and not-very-accurate transcript. It’s a fascinating discussion of a remarkable language and it gives an idea of what we can lose when a language dies. [Edit: Here's a short clip.]