After generating the required locale on our development server and using setlocale
to provide PHP with the locale to use the dates formatted with strftime
It appears that both Apache and Nginx webservers need to be restarted after having locales generated for them to be able to use them without problems.
Either use service apache2 restart
or service nginx restart
to fix the problem. In some cases the entire server might need a restart to fix the problem.
Thanks to Helmut Strasser's blog post we were able to identify the problem.