As far as I know each localization has it's corresponding license key. i.e. you can easily download the localized version from our site: http://www.parallels.com/download/desktop/
However, I doubt that you would be able to use the license key (english version) for french version of Parallels.
I would suggest to create a support request for this matter in our support line: http://www.parallels.com/support/request/
They would be able to provide you a more direct assistance with license key management.
