Ligero vs. liviano
The words 'ligero' and 'liviano' both translate to 'light' in English, in the sense of not heavy, but their usage may depend on the context, region, or the speaker's preference.
Ligero
'Ligero' refers to something being light in weight. However, it also has secondary uses such as referring to someone behaving lightly or frivolously, or food that is easy to digest.
Este paquete es muy ligero.
(This package is very light.)
Tu hermano siempre fue un ligero.
(Your brother has always been light-hearted.)
Después de la cirugía necesitas una dieta ligera.
(After surgery you need a light diet.)
Liviano
'Liviano' generally refers to something being light in weight. It is commonly used when referring to objects or loads. However, it does not have the broader range of use that 'ligero' has.
Esa maleta está muy liviana.
(That suitcase is very light.)
El vehículo se siente liviano cuando no tiene pasajeros.
(The vehicle feels light when it has no passengers.)
Summary
In essence, while both 'ligero' and 'liviano' mean 'light', the word 'ligero' tends to have a broader range of uses beyond merely indicating lack of weight. The level of interchangeability often depends on regional speech norms and variability.