I don't know anything about ESL levels. But I don't think that knowing dirty words is a sign of advanced knowledge of a language. My Chinese roommate in graduate school had lived in the US for over 4 years, and his English was very good. He was a really smart guy, and on most things he could speak clearly and correctly. But we found in trying to discuss abstract concepts that we couldn't communicate at all. Abstract discussions use more general meanings of words, and also much rarer meanings. Words have to be used in certain ways, and those uses aren't necessarily evident from a dictionary definition. So if someone can have a discussion about really abstract ideas, and speak correctly, then I'd say that is a sign of a very high level of proficiency in a language.

" Chino cochino ", I responsed to him: " cállate cabrón" There is no Spanish as second language in Mexico. I learned their language. ESL speakers will never fit into the definition of English speakers. For example: Fxxk you ! you will get a correction from the English speakers, he/she will explains it to you that the subject is missing. English is not the same as ESL. My answer to your question is: The bad or dirty words will not be taught to the ESL speakers. In case, the English speakers call you names. " Thank you sir, or yes, Madam" thats the sole answer that you might have learned. Mr, Wozitoya was my teacher, he taught me read and write in both Spanish and English. I know when, where and how to use the bad words.

Dirty or bad language is certainly not necessary. Whilst most native speakers use it at some point, the majority of educated native speakers will use it sparingly (I also agree with StevenSteven that most educated native English speakers do respect the rules of grammar pretty much constantly). I believe that many of the native English speakers who swear a lot do so because they have a poor command of English. If you are not well versed in the culture of the country whose language you are learning, it is advisable to avoid using bad language. If you do use it, you will probably just end up using it inappropriately or in the wrong situations. I have been teaching ESL for ten years and my advice has always been to, yes, learn the meanings of bad words (because you will definitely hear them), but don't ever actually use them yourself. Hope this helps.

@Robert Yes, but students will learn those expressions pretty quickly when they need to. It's usually pretty obvious when someone is saying something offensive to you, and cinema covers most of the common ones. Regarding using "the bad words". I know many well spoken, native speakers who never swear. So using profanity is entirely unnecessary for speaking "our language". In fact, not using vulgar language is highly regarded by most decent people.

No, because people tend to learn the bad words first in languages. I know a lot of bad words in languages, but don't know any good ones... haha... It is wise to know what someone says to you in a language when it is bad.