Just look at the Date format field in the Preferences screenshot. It's really hard to not find a more user-friendly way to do it, IMHO.

UI and UX matters more than developers care to admit.

Ok, I'll bite-

What would be a functionally equivalent, but more user-friendly way to do it? (And preferably not 10x harder to implement)

For example an additional drop-down menu with common date string formats, where the first entry is the OS default for the given locale.

Additionally one could add a text label, which displays an example date formatted by the currently selected choice.

See? It's really not that hard, you just need to switch the perspective for a minute: what would I prefer if I were a user? Hiding the complexity from the user is one step forward to a better UI.