what are the traits of their formal and in formal uses of languages?

I found some really good excerpts for what you are asking. You can find the source-link at the bottom.

One of the most highly praised aspects of The Color Purple is Walker's use of language. Celie's dialect brings the story to life. Fort (1982) states that "Walker's use of language, especially black idiom, is masterful and adds poignancy and depth to the narrative" (p. 115).

Robert Trowers (1982) in the New York Review of Books agrees, stating that Walker has met the challenge of "the conversion, in Celie's letters, of a subliterate dialect into a medium of remarkable expressiveness, color, and poignancy" (p. 35).

Mel Watkins (1982) in the New York Times Book Review also feels that the language choice of Walker is one of the most appealing aspects of the book. He states that some readers may initially be put off by Celie's dialect and simple way of speaking, but that after the reader accepts the dialect it "assumes a lyrical cadence of its own...The cumulative effect is a novel that is convincing because of the authenticity of its folk voice" (p. 7).