The question of whether or not it’s appropriate to teach ‘colorful’ language is a tough one. Take a
step back from the squeaky clean textbooks and dictionaries that are an inescapable part of language study, and really notice the real vernacular used by native speakers. With your foreigner friends, with other native-speaker teachers, in movies and online videos. Should something so integral to modern English be excluded from study?
As ubiquitous as it is, it’s easy to make an argument that profanity is integral to modern communication and therefore should be fair game for a lesson’s topic. Common words like “sh*t” and “damn” fill many roles in English now, the former easily used to express everything from “assorted things” or “belongings”, to dismay and surprise. Being part of the everyday vernacular, it does a Japanese student of English well to be familiar with the meaning and usage of such terms. In particular, students studying out of love for movies and TV will encounter rough and vulgar language regularly.
However, caution is advised before you start to teach your students how to drop f-bombs like a native. Japan is still a relatively conservative country with a sizable elderly population. As well, many students approach English study as a school subject: scientific and somewhat detached from real world use. Boldly teaching the use of vulgarities without carefully knowing your students could alienate a lot of potential students, or even be a job-ender at some companies.
Perhaps the best tack for this question is to teach such language and terms when they’re brought up by the student or material in question. These kind of naturally-occurring opportunities allow you to broach the topic and teach real English, while maintaining your appearance as a professional and polite teacher. You may find from this that students become more willing to come to you with their questions about other, “relaxed” language or expressions.
But always keep in mind too that sometimes racy language can bring up racy topics, which can easily get
into uncomfortable situations for many teachers.