Actually, there are three countries with similar names that are adjacent to one another in South America:
Guyana is the former British Guiana, and mostly they speak English.
Suriname is the former Dutch Guiana, and mostly they speak Dutch.
French Guiana continues to be a French colony, and mostly they speak French. To confuse matters, in French it is Guyane. That is probably what your French teacher was interested in.
From the wikipedia: French Guiana (pronounced /ɡiːˈɑːnə/ or /ɡiːˈænə/, French: Guyane française; French pronunciation: [ɡɥijan fʁɑ̃sɛz]), officially called Guiana (French: Guyane), is an overseas department and region of France, located on the north Atlantic coast of South America in the Guyanas. It borders Brazil to the east and south, and Suriname to the west. Its 83,534 km2 (32,253 sq mi) area has a very low population density of only 3 inhabitants per km2, with half of its 250,109 inhabitants in 2013 living in the metropolitan area of Cayenne, its capital. By land area, it is the second largest region of France and the largest outermost region within the European Union.
The area was originally inhabited by Native Americans. It was colonised by Frenchmen and other Europeans, who introduced African slaves and later Asian labourers and Hmong refugees from Laos.[3]
The official language is French, while each ethnic community has its own language, of which French Guianese Creole is the most widely spoken.