i18n: add fr-FR translation

Here is the fr-FR translation.

Ordinal indicators (st, nd, rd, th) are not curently translatable. I've added a function depending on the language code.

I'm wondering whether language code could be a widget option. In such case, most recurence.js functions signature would change.
I'm not sur about that : missing ordinal indicator seems better to me than having false one (at least in french). Are you sure you want en-us ordinal indicators, when we know this is not the current language ?