The Toubon Law (LOI no 94-655 of 4 August 1994 relating to usage of the French language) is a law of the French government mandating the use of the French language in official government publications, in all advertisements, in all workplaces, in commercial contracts, in all government-financed schools, and some other commercial communication contexts. This also includes websites.
Find out more at Legifrance.gouv.fr LOI no 94-665
Wikipedia http://en.wikipedia.org/wiki/Toubon_Law#Enforcement_against_French_subsidiaries_of_US-based_organizations gives examples of where an organisation has been fined for not having an english version of their website.
If you already have a website in english, the page layout, colour scheme, etc are already in place. It need not be expensive to have a french version of your french pages. Please feel free to leave a comment or e-mail me if you would like any advice or a free “no obligation” consultation.