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.
As T means either Tuesday or Thursday, we need a contextual marker to provide information about the day the T stands for. So that we can translate it to M in french for Tuesday and J in french for Thursday.

The contextual marker arg is not provided by gettext so we need pgettext. See [contextual marker doc](https://docs.djangoproject.com/en/dev/topics/i18n/translation/#contextual-markers)