> Why duplicate this in Wayland, instead of fixing glibc to provide something sufficient?

Mostly because the date format is a lot more important to the GUI environment (where a human reads the date) than in the low-level stuff glibc cares about. The improvements in graphical environments have sort of moved localization responsibility from the TUI to the GUI. There is less and less incentive to localize the terminal, we are slowly moving to a world where people are giving up on localizing the TUI because technical TUI users can live with C.UTF-8.