There are a few instances of Creole words in the novel, such as when Christophine, Antoinette's nurse, uses the word "beke" for white people. She says,

If béké say it foolishness, then it foolishness. Béké clever like the devil. More clever than God. Ain't so?

However, the narrator gives an explanation for why characters like Christophine speak primarily in English. We learn that she

had a quiet voice and a quiet laugh (when she did laugh), and though she could speak good English if she wanted to, and French as well as patois, she took care to talk as they talked.

In other words, Rhys is following the subaltern characters' pattern of language. This is explained as more realistic than trying to replicate a foreign tongue the native characters would not have used in the context of dealing with white people.

Instead of using a native language, Rhys conveys difference through trying to replicate speech patterns natives use in speaking English, such as when Christophine says,

Woman must have spunks to live in this wicked world.

The question above presupposes that the natives have a "mother tongue," but the reality of the situation is that most natives on the island had their indigenous language erased. Most of the indigenous Dominicans were killed by violence or disease early on after the European arrival. Many who were then labeled native were slaves or the descendants of slaves imported from Africa who had been forced to adopt a "pidgin" language. This was a mix of what they could pick up of the language of the masters, blended with their own language and the language of other captives. Rhys could not have reproduced a "pure" native language.

Also, it is useful to keep in mind that postcolonial literature rejects the idea that there are "rules" that can be imposed from above to dictate how this literature "should" be written.