This is the system Format tab in system settings under Language & Region
I am a Korean American who prefers English and the Metric system.
I want to have a specific system format. An example like so.
Dates. 1990 Jan 17 Mon
Times HH MM SS
Dates & Times YYYY MM DD Name Of Date HH:MM:SS
Currently, you can only use country-specific formats and cannot make ones yourself. This is very annoying as someone who grew up in different countries and travels.
If this feature is not currently available, adding it to a future release might be useful.
Korea typically uses a comma between 4 numbers like nnnn,nnnn.nn not nnn,nnn,nnn.nn
Source: and Korean American and lived in South Korea for 20 years.