Hello. You must use syntax of strftime function (see documentation in link above) which uses percents before letters.
For example "day month(text) year" will be "%d %B %Y". If you don't use percents, function used is date() (
php.net/manual/en/function.date.php), which has different letters and is not translated.
You don't need to set locale to dutch, script does it automatically for every invoice. You only must make sure Dutch locale is installed on server - first by testing it with format above and if result is not in Dutch or dutch characters are not displayed properly, by asking your server administrator.