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.
Why do these need pgettext? I thought that was for handling plurals, which I presume isn't needed here.