The formal greeting is 'Hola, buenas tardes.' while the informal one is 'Hasta luego.' The change in tone is influenced by the level of formality and familiarity in the greetings.
The greeting 'Hola, buenas tardes.' sounds most formal as it combines a friendly 'Hola' with the respectful 'buenas tardes.' On the other hand, 'Hasta luego.' sounds the most informal, as it is a casual way to say goodbye with 'see you later.' The change in tone is caused by the level of formality and familiarity reflected in the greetings.
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