Obviously you should employ some judgement as to when you think it is or is not appropriate to use naughty language, but the ESRB, PEGI, etc have no jurisdiction when it comes to what players do.

That is why in addition to the game's rating, we get the "online interactions are not rated" clause.

More often than not, no one is going to report you for using the f-word or posting a macro of a buff cat with a giant penis - it is still a risk, however. I am more in the camp that people shouldn't be penalized for language unless it is abusive or defamatory, but at the end of the day all of this is at SE's discretion.