Hi,
This is how you should use date_i18n with ACF:
$dateformatstring = "l d F, Y";
$unixtimestamp = strtotime(get_field('date_picker'));
echo date_i18n($dateformatstring, $unixtimestamp);
This depends on your blogs language setting. If you want to manually specify the locale in PHP, ignoring WordPress, you can do it like this:
setlocale(LC_ALL, 'fr_FR');
echo strftime("%A le %d %B, %Y");
//with day of the week = mercredi le 18 septembre, 2013
