Do you know anything about ESL certification?

I am contemplating retirement, and would like to have something to occupy my time. Teaching English as a second language seems like it might be worth doing. I did a Web search for certification training, and came across several for profit colleges, which made me skeptical of what they have to offer.

It seems that ESL is such a broad area that the best that anyone could offer as preparation would be a single course giving a broad outline of approaches to take for different levels of student.

Do you have any experience teaching ESL or know of someone who has?