You Do Need power plug adapter while traveling from Nauru to Cuba.

Here is Why You Need An Adapter

The outlet and plug type used in Cuba are A, B, C and L and in Nauru is I. This is how they look like.

Nauru

In Nauru, outlets of type I are in use.

Type I

Cuba

In Cuba, outlets of type A, B, C and L are in use.

Type A

Type B

Type C

Type L