Using Google Translate against your post on this English only forum:
Hi, I'm trying to install a Fiery driver on a Mac OS 10.7.5, but I still have the same error message, which tells me that I have to change the language of the OS to English to install. I tried, it does not change anything, always same message. I downloaded the driver from the Fiery, from the Xerox site and also from the site finally. I always have the same message. I have another mac with the same OS and the installation went well. If anyone has an idea, she is welcome. thank you in advance
Assuming you completely uninstalled the driver with the Fiery drival removal tool that EFI gives out with their download, and then rebooted the Mac then changed the language and rebooted again and then reinstalled the driver, you will need to contact EFI directly.
But since you posted in French, it is an odd message to get since the driver fully supports French assuming you are using the EFI version and not one from Xerox.com for an English location.