You Do Need power plug adapter while traveling from Monaco to Belize.

Here is Why You Need An Adapter

The outlet and plug type used in Belize are A, B and G and in Monaco are C, E and F. This is how they look like.

Monaco

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

Type C

Type E

Type F

Belize

In Belize, outlets of type A, B and G are in use.

Type A

Type B

Type G