R – how to localise a sharepoint site for different languages

If it is just a matter of migrating files, you could actually use the content migration framework. The core classes of this framework are the SPExport and SPImport classes in the Microsoft.SharePoint.Deployment namespace. You can with fairly little code export individual files to disk and import them again to a new location. Alternatively, try the SharePoint Content Deployment Wizard on CodePlex here: http://www.codeplex.com/SPDeploymentWizard. It will save you the coding.

Changing the language of a site after it is created is not really supported, but I did find this blog post that tells you how to do it by cracking open the DB (I do not recommend this approach).

The better option is to delete your French variation and then recreate it. When you create the label be sure to leave the English language selected. This tells SharePoint what language pack to use when creating the site.