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

Here is Why You Need An Adapter

The outlet and plug type used in South Africa are C, M and N 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

South Africa

In South Africa, outlets of type C, M and N are in use.

Type C

Type M

Type N