Arguments about the need for national or cultural identity are often seen as being opposed to those about the need for mutual intelligibility. But this is misleading. It is perfectly possible to develop a situation in which intelligibility and identity happily co-exist. This situation is the familiar one of bilingualism – but a bilingualism where one of the languages within a speaker is the global language, providing access to the world community, and the other is a well-resourced regional language, providing access to a local community. The two functions can be seen as complementary, responding to different needs. And it is because the functions are so different that a world of linguistic diversity can in principle continue to exist in a world united by a common language.
– David Crystal, English as a Global Language (1997)