Nativity is not necessarily a sine qua non for teaching a language as most Chinese have erroneously conceived. In fact,teaching English (which is one of the most widely spoken languages around the globe) is a question of who is a qualified language teacher. To be a qualified language instructor would mean, to master and command the structure and aspects of a language and be able to effectively transmit it to a leaner in a manner that he assimilates it thoroughly.You may come from America, the UK,Australia Canada and New Zealand but may not necessarily be a qualified teacher of English,though you may be qualified to teach some other subjects in English.There are qualified teachers of English in native countries as well as unqualified ones and this applies to non-native speakers also.That explains why some schools will prefer English teachers from some countries only;say Canada or American,etc and some will not even specify.Furthermore, that is also the main reason why some teachers excel in their teaching than others.