Can I show a 24-Hour clock on my Mac's Lock Screen?
Since at the login window (and the lock screen) your user is not logged in, you´ll have to edit the global preferences: 

sudo defaults write /Library/Preferences/.GlobalPreferences AppleLocale en_GB


and:

sudo defaults write /var/root/Library/Preferences/.GlobalPreferences AppleLocale en_GB


and then do a reboot. This fixed it for me (and I hope, Apple fixes that sooner or later). :-)