Martinique is significant to Wide Sargasso Sea as it is where many of the servants in the novel come from. Its first mention is on page 1 of the novel where Rhys writes "She was my father's second wife, far to young for him they thought, and, worse still, a Martinique girl" (page 9 in this edition of the novel) in reference to Annette. Martinique is mentioned sixteen other times in the novel as where Christophine is from (Rhys, 11), where Antoinette gets some of her dresses or headscarves (Rhys, 12), and Antoinette even refers to St Pierre, Martinique as "the Paris of the West Indies" (Rhys, 47). The French-sounding name and the comparison to Paris make sense when considering that Martinique was colonized by France and, even with Britain taking control of the island on multiple occasions, remained linguistically and culturally French. This explains Annette and Antoinette's French names. Aside from that, the history of the island relates back to the power of names and the themes of colonization in the novel. Antoinette's French name is changed by her unnamed husband to the good English name "Bertha", mimicing how Martinique was under British control. However, Antoinette was still Antoinette to herself and always referred to as such in the narration.
Sources:
https://www.martinique.org/en/about/history
