I’m using a slightly modified locale on my Mac because I like everything to be in english1. However, I do not like the english date formats using AM and PM2, so I fixed that by customizing the locale.
Unfortunately this customization doesn’t fix the clock on the login screen, but this neat little shell command does:
Please not that this might also change your keyboard layout, which mean that some keys and symbols might not be where you expect them to be.
Fortunately you can turn on the Input menu in the login screen in
System Preferences ➡
Users & Groups ➡