You Do Need power plug adapter while traveling from Liechtenstein to El Salvador.

Here is Why You Need An Adapter

The outlet and plug type used in El Salvador are A and B and in Liechtenstein are C and J. This is how they look like.

Liechtenstein

In Liechtenstein, outlets of type C and J are in use.

Type C

Type J

El Salvador

In El Salvador, outlets of type A and B are in use.

Type A

Type B