Because it is a French Department. Kind of like America owns places in other parts of the world.
78.6% of the people in French Guiana speak French...
French Guiana
No one France owns French Guiana and they speak French there, too.
The population of French Guiana speaks French.
They speak French in Guiana. (wow the last guy wrote France.)
French is not a language in Guyana. You are likely thinking of French Guiana, which is a different country.
Only one: French Guiana.
100 percente
French Guiana was once a colony of France, so they speak French.Suriname was once a colony of the Netherlands, so they speak Dutch. Guyana was once a colony of Great Britain, so they speak English.
Brazil, French Guiana
French Guiana is the only country in South America that speaks French.
French Guiana (Guyane Française) is a french colony. So, that's 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.