You Do Need power plug adapter while traveling from Mongolia 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 Mongolia are C and E. This is how they look like.

Mongolia

In Mongolia, outlets of type C and E are in use.

Type C

Type E

French Guiana

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

Type C

Type D

Type E