Questions & Answers about Noi mergem la un restaurant liniștit diseară.
La expresses movement to a place (a restaurant, the cinema, school etc.). Using la is the standard way to say “to [that place]”:
– la restaurant – to the restaurant
By contrast, în would mean “into/inside”:
– mergem într-un restaurant – we go inside a restaurant (less idiomatic for “we’re going to eat there”)