I think it really depends on the ref. Just last summer I was playing a game and one of my team mates yell leave....ref blow for a foul, mind you, he was a Trini ref. I think the general rule up here in Toronto is that if you do say leave, its a foul.
It is unfair though because leagues here are multi-cultural and we have a lot of Greek, Italian, Russian, Croatian and Spanish teams, so unless the ref is versed in multiple languages, there is no way he would know when that is being used.
Generally they tend to say John's ball etc.