English does have double negatives, and they're grammatically correct when used properly. They simply cancel each other out which differs from Spanish where they add emphasis.
For example, in my native dialect, "I ain't never had no need for none of them fancy toys" means "I never had any need for any of those fancy toys". It has nothing to do with canceling out other negatives or emphasis; if one thing is negative, they all need to be.
They don't! "I ain't got no bread" means "I haven't got any bread". If they cancelled out it would mean "I have bread".