Diction refers to the word choices an author makes, and diction exists on a spectrum from formal to informal. The dominant speaker in "Girl," evidently an older woman with some authority over the titular girl to whom she speaks, uses informal, often colloquial diction. She discourages the girl from singing "benna" in church, teaches her how to make "doukona," and advises her not to act like a "slut." Benna is a type of calypso-style music popular in Antigua and Barbuda (often with salacious subject matter thought to be inappropriate in a house of worship); doukona refers to a Jamaican and West African dish called pepper pot; and slut is an informal word used to insult a female who is believed to be sexually promiscuous.
Colloquial references to benna and doukona, for example, help us to understand where this story is set: in the Caribbean. The girl's responsibility for cooking family dishes helps us to understand her role as a glorified domestic servant rather than as a partner in a relationship. The primary speaker's constant references to the girl's desire to become a so-called slut also help to show us that much of a girl's value rests on her reputation and her sexual innocence before marriage. These diction choices help Kincaid to convey the message that girls, perhaps especially girls in the Caribbean, are typically and unfairly made to understand that their social value lies in their domestic service and their virginity.