As CELTA course director, I often get calls from potential CELTA applicants asking whether CELTA is only for native speakers of English. The easy answer is NO ….
Anyone who has a good C1 (CEF) level of English can apply to take a CELTA course. The application process tends to whittle out the applicants whose level of English is not good enough to successfully complete the course. At our centre, we will not offer anyone a place on our course unless we are certain their level of English is good enough. During teaching practice, our trainees teach at A2 (elementary) and B2 (upper-intermediate) level, should our trainees’ English not be much better than some of their students it would be more than embarrassing for all concerned.
There are some advantages of being a non-native speaker, the biggest of which is that you have already gone through the language learning process yourself and are therefore better equipped than the native-speaker. Non-native speakers tend to have a better language awareness than native speakers. Have a look at this previous blog post CELTA and non-native speakers of English for more on this subject.