The clothing is western-style (based on French fashion), but adapted to the tropical climate.
The men wear shirts and trousers, while women wear blouses and skirts or cotton dress, or trousers.
Why do they speak French in French Guiana?
French Guiana (Guyane Française) is a french colony. So, thats why they speak French there. Local dialects may be spoken through the region, but French is by a large margin the main language in French Guiana, and the one used in schooling and administration.