Global 24-hour time format in KDE Plasma
Solution 1:
In KDE, go over to the clock on your bottom taskbar and right-click it. Once done there is a coniguration icon.
Once done click on the Time-Display option and then you can change the time-format to either 12-hour or 24-hour. Hope it helped.
Solution 2:
In KDE 5.22.4 (Find your version in the About program):
Right click the time > Configure Digital Clock > Time Display > 24-hour