phranque - 2:50 pm on Feb 17, 2012 (gmt 0)

it partly depends on whether you are targeting a "language" (french) vs geotargeting (france).
i would usually prefer to specify languages in the hostname as subdomains.
this makes it easier to host a subdomain in the targeted geography should that become necessary.
it also makes it easier to have the same directory structure for all languages without the requirement to specify a default language.
e.g. www.example.com/category/product vs en.example.com/category/product

if you are using a gTLD make sure that you specify a url structure that allows you to geotarget in GWT as required. (i.e. subdomains or subdirectories of the document root directory)

regarding usability, consider how you will address default and preferred language detection and provide for switching languages.
this includes detecting the Accept-Language‎ HTTP Request header setting and perhaps the existence of a language cookie from a previous visit and redirecting to the appropriate url if necessary.