A friend of mine complained that his eleven- and thirteen-year-old foster sons frequently spiced their talk with crude sexual and violent words. “They use the f-word and speak about killing some “dickhead.” Usually it’s coupled with a mean putdown. I think they’re mainly doing it to shock my wife and me.”
Of course they are trying to shock you. Perhaps to show with their language you how tough and independent they are. So try not to overreact. Words themselves aren’t bad. Actions can be.
I think the biggest mistake you can make is to respond with a long lecture, explaining why those words are inappropriate and wrong. And likely to get them in trouble. The main problem with lectures is that they take too long. By giving attention to the very behavior you wish to discourage, your reaction provides the reaction they were seeking in the first place - attention.
Threats and punishment are also unlikely to be effective. They are more likely to involve you and your foster sons in an ongoing battle about how to irritate each other. With teenage boys, you likely have bigger issues than bad words that require correction.
If you can, ignore the words. Ignoring is not doing nothing. It may take a bit of time, but your failure to react or become upset is a good way to get rid of the undesirable expressions. If you ignore something, it is likely to go away.
You might try humor. Together, invent some nonsense words that might be used to replace the crude ones and still express the strong feelings. Or hold your nose and look sour. I had another friend who began to laugh, telling his foster son: “You just set a new family record, using the f- word eight times in one sentence. Congratulations. I thought that was impossible.” The young man was not expecting his crudity to be treated as a joke. He never used the word again.
Finally, set a good example. Don’t salt your own speech with language or putdowns that you don’t expect to hear from your foster sons.