Well... I wrote another reply before but I deleted it later because the thread was in Basses forum, now as it is in Misc. I can write more.
To know french has an effect but not very big, the way the sellers treat you changes between worse and worst

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Isn't it a nice excuse? They don't put gun on your head to work there. Or do they?
Wow, isn't it nice to educate rough people! Customers, be nice so that his majesty seller will treat you well, if not, they'll treat you like shxt because there are lots of tourist there, they bore those highnesses, so if you say hello (en français, bien sûr), smile, offer a glass of wine they may treat you NORMALLY...
I don't advocate being rude but to be treated as a customer should be treated normally, you shouldn't have to make so much effort. An average manner should be enough to be treated the way it should be. BTW, I always act this way everywhere (not trying to be nice so that his majesty will treat me well) because I'm most of the time a polite and nice guy (would you believe it?). And believe me, the result was not very different when I was in Paris (Bonjour, je voudrais acheter des cordes pour basse et si c'est possible j'aimerais bien jouer de ce Fender).
I've been maybe over 15 different countries so far (can make comparison), I'm francophone, I studied 8 years of French with French teachers, I worked in a French Holiday Village for some time, I still have communication with French people in my current job. So I can say that I know less or more French people. And all I can say that they are not famous of their hospitality. I wouldn't call them polite either, if you'd like to see polite sellers, see Vienna, see for example SE of the States. For Paris, it is a nice city, with beautiful buildings, churches, gardens, museums (but I'd rather be in Roma or in New Orleans that I find more beautiful). And for buying instruments, bass related things, nothing comes close to the States.