Facilitate means 'to make something easier'. Here it means that other people make it easier for English speakers by speaking English instead of making the English speakers speak and listen to a different language. Another way to write the sentence could be: Because English is studied by so many people, in so many countries, people who speak English as a second language will usually speak English to make it easier for the native English speakers, while native English speakers do not usually try to speak other languages to make it easier for people who don't speak English very well.