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

Here is Why You Need An Adapter

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

French Guiana

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

Type C

Type D

Type E

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G