Well, that may be a literal word-for-word translation, but the French for "brass" is ALSO cuivre (or specifically, cuivre jaune). Brass, of course, is an alloy that contains copper at varying levels.
The name for "the brass (instruments)" in French is les cuivres.
As for the dial you are seeing, I am unable to find that on my copy of Sibelius 6, running under either the English or French localization. Can you provide a screenshot of the setting in context?
After seeing this in context and doing a bit more research, it appears that in addition to the language definitions above, cuivre is also used as an indication for brass players to play with a "brassier" tone (i.e. harsh or overblown... you'll know it when you hear it--think drum corps. playing). So yes, it is a change in the sound, although not directly related to the proportion of copper in the brass alloy in this case -- just an emulation of a slightly different playing technique that results in a desired tone.
The Sibelius 7 instrument library pulls this property out of the regular instrument into one that can be adjusted independent of other variables (in older versions, you would expect to hear a brassy or overblown tone come into effect with an unreasonably high dynamic level like fff.)