You Do Need power plug adapter while traveling from Balearic Islands to Saint Lucia.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Lucia is G and in Balearic Islands are C and F. This is how they look like.

Balearic Islands

In Balearic Islands, outlets of type C and F are in use.

Type C

Type F

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G