The laws used to say foul and abusive language, but I think that has changed now to something more subjective, like wilfully abusive, or similar.
I remember refereeing a game in Beswick or Collyhurst or somewhere. One guy in particular used a derivative of "f*ck" in nearly every sentence that came out of his mouth. He would criticise me using the word. He used it towards his teammates, his manager, opponents, everyone. Should I have sent him off for (incorrectly) calling me "f**king useless"? Maybe, or perhaps I should have cautioned him for dissent.
I didn't though. It made me think that although this was quite coarse language to my ears (strict Christian upbringing, amongst people who never swore), this was just everyday conversation to this player, and he would talk like this to his wife, kids, parents, employer, friends etc. It wouldn't have been fair of me to apply my own standards to him.
There are plenty of laws of the game that aren't applied correctly or consistently though.