It's just a very informal way of speaking. I hear this construction often and use it sometimes myself, but it's frequency probably depends a lot on location. Some informal versions of English would use it more than others.

We don't miss out auxiliaries. 'You speak English' is completely wrong. What you may have heard is something like 'D'you speak English?' where the 'Do' sounds like it is combined with the 'you', but the auxiliary is still there.

I guess that the answer is the same in any language. Laziness, ignorance, anarchy, familiarity... I would never say that, since if I felt the need to ask that, there was a good chance that they didn't, and I would not want to confuse them...