At the top right of the Market-Q login page is a pulldown which allows the French Language version of the site to display on Market-Q sites and co-brands which offer that version.
Once 'FR' is selected, the login page will appear in French
A quickstart guide in French can be found here
As will most of the menu and window labels, with the exception of user generated labels such as found on the watchlist, which will remain in whatever language they were entered by the user as well as News Articles and Most Market Research.
Under Utilitaires->Preferences the Preference Menu will show a Language Option for those sites and co-brands that support it.
If the Change is made it will happen immediately.
A user will be given the same language to logon by default that they last used during the previous session as long as the same browser is used and the cookies are not cleared.