This is a great question. I would say that the use of language was brilliant. In particular, I can think of two ways.
First, as you probably know, back in the early 1900's, the world was highly stratified in terms of social class. The way that the people spoke in the movie made this very clear. Those of the upper class has a certain way of speaking. Their accent was different as well as their vocabulary. Hence, we can say that this aspect made the social realities of the time come to life. Cameron is to be commended in this respect.
Second, the character, Jack, though poor, also had a great command of language. He did not have the pretentious style of the others, but he spoke "truth." We can say that his words broke through the thin veneer of superficiality. This was one of the reasons why Rose fell in love with him.