The GNOME shell offers no options for customizing the default system locales. You can create your own C locale setup, but this requires significant work and know-how. If you just wish to customize the GNOME shell top-bar clock; there are easier options. You can use the Clock Override extension for GNOME shell and have absolute control over the primary time and date display in GNOME. The extension includes all the standard options you'd expect plus some unusual ones like a Unicode emoji clockface and time-keeping using .beats Internet Time.