Compatibility summary
You will need a travel adapter in El Salvador; plug types used there differ from France. Voltage differs by more than 20% — a voltage converter may be needed for some appliances.
Plug comparison
France plug types: C E
El Salvador plug types: A B
There is no common plug type between France (C E) and El Salvador (A B). You need a travel adapter.
Voltage comparison
France: 230V, 50Hz
El Salvador: 120V, 60Hz
Voltage converter recommended (France 230V vs El Salvador 120V).