This should be a global setting in SmartThings. In fact, the SmartThings app used to follow your mobile device’s 12/24 hour clock setting.
I see you’ve posted in a thread regarding this, saying that a support ticket should be opened with SmartThings. Did that happen?
I think it is a way better approach for SmartThings to implement it again, and then device drivers / SmartApps can implement code that uses the system’s settings for 12/24 clock, rather than every device driver / SmartApp having to include a preference setting and use its own method to display 12/24 clock.