I want to have it in the 12 hour format but It's stuck in the 24 hour format, the Steam settings do nothing, and it doesn't matter if I use the flatpak version or the apt version as they both have the same bug. Zorin is set to the AM/PM format in my Date and Time settings.
Did You tried it with a Reboot?
If you mean rebooting my system it's the same every single time. I've even tried changing locales just to see if it would fix it but it seems like it's just stuck in the 24 hour format.
Have you completely killed all Steam processes and then in terminal run
sudo update-locale LANG=en_US.UTF-8
In Steam > Settings > Interface, have you checked that it matches your local region?
My Steam client language is set to English is the interface settings. I've tried doing that in the terminal after killing Steam and it doesn't fix the issue unfortunately. I've tried both the stable version and beta version of Steam but they have the same issue.
I've been looking into this a little bit here and there and it seems to just be a common issue with steam period. I've never been able to get it to functionally work, regardless of Linux or Windows. Seems like it's inconsistent too. Many reports of it not working with no fix, and some report that it sometimes works.
Unfortunately I don't think a nice fix is going to come here. It seems like this may be a Valve problem that they might need to fix.
EDIT: Just moving the thread from general to gaming, as it's more pertinent to that I feel.