Denis's French is correct (and phonetic spelling ok).
But if you are actually trying to 'negotiate' a sale at a Paris BMW dealer, that level of French will not convince any salesperson you are French (not American).
I suspect most Paris BMW sales staff understand/ speak English better than that level of French. If so, you may as well use American English.
If the issue becomes: "will you sell this System 5 to me as an American [since that's what I am, obviously]" and the BMW dealer refuses/ is reluctant because you are American, you could try offering to pay directly in cash Euros (not US credit card or Traveler's checks). A cash sale leaves no paperwork trail that might come back to haunt the dealer. Then, the dealer has to balance the value of the sale vs. very unlikely consequences of "disobeying the BMW policy."
Let's be clear: there is a BMW corp policy/ embargo against selling System helmets to US citizens. The policy was first imposed in ~ 1998, with intro of the System 4. The BMW Legal Dept imposed this policy on BMW Motorrad & all retail dealers worldwide, because of the Legal Dept's assessment of possible legal risks by (us) litiguous Americans.
However, that policy is not necessarily well-known/ re-enforced at BMW retail sales level, and even when known, is often ignored. It all depends on local country & dealer circumstances at the specific time you are attempting the purchase. Many US citizens--including me--have bought System helmets overseas without hassle or with minimal hassle. I have even had a BMW employee-salesperson at a major BMW-company owned dealership in Germany offer to sell me a BMW Carbon helmet if I payed in cash (ATM across the street) so there was no paper trail.
It's all fairly stupid....just realize, you takes your chances in trying to buy
Also--don't forget, the System 5 is a good helmet, but it ain't necessarily worth a lot of extra effort to obtain just because it's "forbidden." Most important is that a helmet fits you. Beyond that, many helmets are of "equal" quality & more easily obtained (for Americans) than the System helmet, including Schubert, Arai, Shoei, jadda jadda.