They also say that it can be something at the OS level, which I would also agree with.
You can see the available locales on your server with `locale -a`
To test whether a locale is supported, we try setting the locale (via setlocale( LC_ALL, $locale ); ) the various possible names of the locale (some systems have .UTF-8 or .UTF8 in the locale name). If this fails (at PHP/OS level, i.e. the locale isn't present or working), it won't be available as an option on the list, or usable as a custom definition.