English is not the world official languages, there is other nationalities and people over there ! And there is many people who can speak English better than many natives English speakers. So how will you know ?
English is the language used for the financial services in most of the Western world. It is also really annoying to try to read unintelligible posts on the English boards. It really slows down the pace of thread scanning. Bitcoin Talk has a number of boards for a variety of languages - it makes much more sense to post in a language in which you have some fluency, rather than clogging an English thread, and being ignored or flamed.