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

Here is Why You Need An Adapter

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

Belize

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

Type A

Type B

Type G

Mongolia

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

Type C

Type E