I would go with option a.

Mainly because it makes sure your domain is a valid page in it's own right and there is no duplication of your French content.

This stuff will also help you tell search engines what you are up to:

[support.google.com...]
[support.google.com...]

It's a good idea to include the language identifier in the URL.

Faced with a multi-language site where the information I wanted was in Armenian, right from the Google search I made, substituting /en/ for /hy/ brought me to a quaintly worded but quite understandable version of the data I was looking for.

Thanks to both of you for your replies.
However, it would seem that you are not in agreement.

Tiggerito is saying to not use systematically the language identifier for the domain entry page.

g1smd is saying to use the language identifier all the time.

So which is the "correct" way of doing this ?

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.

Ok phranque, you really didn't want to make it any easier ! ;-)

My site already handles the two languages via data stored in a cookie, with an on-page language selector. This is "simply" a language choice and not a geolocation choice, enabling non-french speakers to still be able to benefit from the content.
Using a sub-domain approach would indeed ensure that the existing functionality of the site would be left untouched.
However, on a LAMP hoster with CPanel, I am unsure as to how to set this up without duplicating the physical file data.
Any advice ?

this is the CPanel doc for setting up Subdomains:
http://docs.cpanel.net/twiki/bin/view/AllDocumentation/CpanelDocs/SubDomains [docs.cpanel.net]

if you are hosting all the content in the same document root, you can do any necessary redirects and/or internal rewrites for all subdomains in the .htaccess file there using mod_rewrite.

oh and i didn't notice earlier so welcome to WebmasterWorld, Simon_Giddings!

Sorry phranque, still a bit confused.

Does this mean that I should create the subdomain, pointing it towards the same doc root as the main domain ?

I think I can see what I would need to do within the .htaccess file.

yes that's what i mean, assuming "you are hosting all the content (both languages) in the same document root"