Depends on the history. You never fire somebody on the first offense in a case of using inappropriate language on the job, because you have to account for their personal background - maybe where they grew up that language is totally normal and they really don’t get that it’s a problem. They have to be given a chance to change their behavior once they’re aware that it’s not okay. If they do it again, it’s a valid violation of workplace standards and totally justifiable to fire them.