Funny, they kinda use the same word in French for lightsabers as in Italian : "sabre laser", which translates to... Laser saber.

Doesn't quite have the same ring to it.

Like you said, "Sabre de lumière" in French just sounds funny and long.

The funny thing about the French dubbing of SW is that they went out of their way to change characters' names and the name of ships and other things to make them sound more "Frenchy" for the first film... And then decided to do a kind of 180 for Empire and made the translation more "literal". They even changed the dubbing actor for Darth Vader for the next two films. I personally hate D.V.'s voice in the first film (sounds flat and the voice effect they use sounds cheap) but other people disagree. You decide:

http://youtu.be/NUtRdVMqErY?t=4m12s

http://youtu.be/_m7JifBw0CM?t=6m27s

http://youtu.be/UQVV-IW2Vsc?t=42s