how does their spoken language differ from what we consider to be Standard English?

Even during the depression the social status of George and Lennie was low. They were itinerant labourers. This comes out in their grammar and diction. When George alludes to not having tomato ketchup he says, "Well, we ain’t got any" . The grammar is of the uneducated in that the use of "ain't" for "do not" is used throughout the novel. Similarly, Lennie uses the grammar the working class combined with that of a child, "Tell about what we’re gonna have in the garden".

Note -I used this response again because it worked within the same context as your last question.