Check locale availability upon configuration save

When the locale chosen for FD is not installed on the server, setting the locale fails and FD triggers a PHP error to signal the problem.

I think it would make sense in the configuration screen to check if the locale chosen is available in the check() method, therefore forbidding the admin from selecting a locale that will cause problems.

This does not fix the case where locale is set to automatic and the user browser asks for a locale that is not set up, but it will help with other cases where a locale is chosen in configuration.