"Beautiful" is surely included in the meaning of 「うららか」, but its more important base meaning is "spring-like" ("printanier" in your language). In the world of haiku, 「うららか」 simply means "spring" itself.
A very beautiful day in any other seasons would usually not be described as 「うららか」by native speakers regardless of how beautiful the day is. It just feels wrong to do so.
It is such an elegant word that it is difficult to translate except for saying that it is along the lines of "peaceful, beautiful, refreshing and spring-like".