Hello,
Usually its not a problem, and answer is YES.
Depends on how website is made.
Most CMS has build in support for many languages - you can choose language while on webpage, your selection is being saved in browser cookies or in website database depends on implementation (so when you get back on page you will see it in desired language).
Basicly when page is generated dynamically, proper XML file is loaded and button name from french pack is loaded instead of english. It doesnt make any changes to webpage itself, just change text on buttons etc.
And thats how it should be done properly.
If french and english are separate websites, you can make shared database for login system, so that both sites have same users with same passwords, etc.