Created attachment 192685 [details] language setting I have "english" as interface language and "german" as locale setting. Despite of the locale setting the margins are shown in inches instead of centimeters.
Created attachment 192686 [details] margins in inches (")
What LibreOffice version and operating system even? i.e. on Linux I have UI language en-GB, locale en-CA, but LC_PAPER=de_DE.UTF-8 and I correctly get A4 with width/height/margins in centimeters. Looks like you have LC_PAPER inherited from some en_US.UTF-8 locale or such.
Or rather en_GB.UTF-8 because with en-US it would be Letter size paper.. but then I saw the Hardware/OS field set to Windows so this is probably all moot.
Even better, there's Tools -> Options -> Writer -> General, Settings, Measurement unit. That switches exactly that. I guess we can close this when you confirmed.
It's hidden and a special treatment for the measure unit. I think that it also should be determined by the locale.
In any case, it could be a request for enhancement but not a bug.