You Do Need power plug adapter while traveling from Congo to Tonga.

Here is Why You Need An Adapter

The outlet and plug type used in Tonga is I and in Congo are C, D and E. This is how they look like.

Congo

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

Type C

Type D

Type E

Tonga

In Tonga, outlets of type I are in use.

Type I