If you search on the internet you'll find a lot of explanations about why the french title was translated to "La Guerre Des Etoiles" instead of "Les Guerres De L'Etoile".
Nowadays, almost everybody says Star Wars. When people are talking about the trilogy or the whole saga, they say Star Wars and specially the younger people. Only some nostalgic people like me or maybe marvins keep saying La Guerre Des Etoiles.
Marvins is right, I remember about that too, when i was a kid we only used to say "LGDE" to talk about the first movie. For the Ep.V & Ep.VI we used to say "L'Empire Contre-Attaque" and "Le Retour Du Jedi".
Even if i was born much much later than marvins i remember seeing the trilogy before they started to destroy it in 97.