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.
This is presumably only undefined if someone has overridden the widget JavaScript? If so, let's just break that for now, and mention that change in the release notes (since I think it's very unlikely anyone has done this).