The posting by Ron Edwards was useful. It misses out a qualification that has international recognition - CELTA (Certifcate in English Language Teaching to Adults).

Although Ron's list is quite extensive and explanatory, not all teaching 'English as a foreign /other language' qualifications have the same standard or rigour. Having said that, my own experience is that demand for native English teachers outstrips supply in Asia to such an extent that a considerable number of working teachers don't have any specialist qualifications or experience at all. Is this a disadvantage? Judging by some of the people I have met and worked with, on the whole I would say 'Yes'. They wouldn't know a homophone from a synonym, let alone how to teach pronunciation. Are they bad teachers? Well, it's quite simple really, if they can keep the students entertained by showing lots of (pirated) American movies and pass students who would normally fail, everyone is happy:the institution moves up the rankings table and attracts more students due to its success rate(sic).

Having actually trained teachers in a past career, there are those I met who were limited academically but turned out to be really excellent teachers. Horses for courses?