Maybe QT doesn’t think linux is not “Linux Is Not UniX” enough. Multiple bug reports on this in the QT’s tracker. A 2017 comment from QT, in a naive interpretation, essentially amounts to QT’s mainly targeting application development rather than desktop environment or operating system development, so instead of QT’s changing the API to accept a handful of custom locale formatting strings (which is the way to go), Plasma should stand up if it’s more than an application and parse langinfo itself for QLocale to use it, since Windows and MacOS are already doing it for themselves in a CLDR-compatible (Common Locale Data Repository) way. A 2024 comment from QT even talks about “The Free World”. Strange stuff in the programming world these days :]
You can try System Settings > Region & Language > Time > Modify: "English (South Africa)", which corresponds to the en_ZA.UTF-8 locale, and uses the format “Saturday, 25 May 2024 17:31:38 GMT+3 | 2024/05/25 17:31”. If a short date format that facilitates sorting isn’t preferred, “English (Denmark)” can be selected, which corresponds to the en_DK.UTF-8 locale, and uses the format “Saturday, 25 May 2024 17:31:38 GMT+3 | 25/05/2024 17:31”. “English (Switzerland)”, which corresponds to the en_CH.UTF-8 locale, and uses the format “Saturday, 25 May 2024 17:31:38 GMT+3 | 25.05.2024 17:31”, is the only locale that uses the period character as the delimiter in the date, although it isn’t a standard locale installed by the glibc package on Arch Linux and the glibc-locale-source package on Fedora Linux.