Par Danny Hieber, via Mises.org:
The history of the world's languages is largely a story of loss and decline. At around 8000 BC, linguists estimate that upwards of 20,000 languages may have been in existence. Today the number stands at 6,909 and is declining rapidly. By 2100, it is quite realistic to expect that half of these languages will be gone, their last speakers dead, their words perhaps recorded in a dusty archive somewhere, but more likely undocumented entirely.
What causes this? How does one become the last speaker of a language, as Boa Sr was before her death in 2010? How do languages come to be spoken only by elders and not children? There are a number of bad answers to these questions. One is globalization, a nebulous term used disparagingly to refer to either global economic specialization and the division of labor, or the adoption of similar cultural practices across the globe.
The problem with globalization in the latter sense is that it is the result, not a cause, of language decline. Another bad answer, encompassed in the former definition of globalization, is trade and capitalism. Trade does not kill languages any more than it kills any other type of cultural practice, like painting or music. Trade enhances the exchange of cultural practices and fosters their proliferation; it does not generally diminish them. Historically, regional trade has fostered the creation of many new lingua francas, and the result tends to be a stable, healthy bilingualism between the local language and the regional trade language. It is only when the state adopts a trade language as official and, in a fit of linguistic nationalism, foists it upon its citizens, that trade languages become "killer languages."