Ok, so let’s say ‘data_picker’ is your date field name in ACF. Set the locale before you go into the loop:
<?php setlocale(LC_ALL, 'fr_FR'); ?>
Then inside the loop do these steps:
// FIRST, SET THE WAY YOU WANT TO FORMAT DATE
// DETAILS HERE - https://php.net/strftime
$dateformatstring = "%A le %d %B, %Y";
// THEN, CONVERT THE FIELD FROM ACF TO UNIX TIMESTAMP
$unixtimestamp = strtotime(get_field('data_picker'));
// NOW ECHO THE FRENCH DATE USING PHP'S strftime
echo strftime($dateformatstring, $unixtimestamp);