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

Here is Why You Need An Adapter

The outlet and plug type used in Saint Helena 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 Helena

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

Type G