Hi Michael
I'm not quite sure what you want to know.
I'm sure that many/most language schools will provide you with some sort of document stating that you attended and successfully completed certain ESL coursework.
However, if you have to prove the level of your proficiency in English (for admission to a university, for example), then you should take a test such as TOEFL or one of the Cambridge ESOL exams.
Or do you want to know about being certified as an English teacher?