You Do Need power plug adapter while traveling from Mauritania to French Guiana.

Here is Why You Need An Adapter

The outlet and plug type used in French Guiana are C, D and E and in Mauritania is C. This is how they look like.

Mauritania

In Mauritania, outlets of type C are in use.

Type C

French Guiana

In French Guiana, outlets of type C, D and E are in use.

Type C

Type D

Type E