harley: I agree.... but I also think that in either sentence the accent is on the word English, not teacher. If the accent were meant to be that you are a teacher, you would simply say, "I am a teacher". Its likely that if you were in a foreign country , you might be trying point out that you are a teacher from another country. But if I am an American teacher in England and I say " I am an American teacher", you will not be confused as to what I meant. Basically the only time there will be confusion is if a person with an obvious British accent is in America, and says "I am an English teacher". But its likely that he would say "I am an English teacher from England".

Point is that in almost every situation, if someone says "I am an English teacher", they mean that they teach the English language, regardless of which word is accented.