Hi,
Disclaimer: I am fully aware that your Windows Server support is very limited. I do not have any problem with languages. I'm reporting just in case:
I assume that the selector should show only locales which are already installed. But on Windows Server the selector seems to work the way around showing only not installed locales. On my Windows Server 2019 which supports out of box most locales the menu includes only very exotic languages. For example, no English (United States) available to be selected while my OS is a native en-US.
So for all locales already supported by my server I need to select "My locale is not listed" and input a correct short code. Following the Windows tip from the PHP setlocale description that would be this format (eg. "usa" instead of "en_US" but it can't be "enu" because then a flag is not detected in the Languages menu at the bottom).
As I said, currently I don't have any problem with languages. But is this a bug in the selector?