the default is usually "C" or "POSIX" which is set by Windows explicitly in the Windows libraries
Correct, so you call `setlocale(LC_ALL,"");` to select the default locale.
#1757, #1919 and some others show that Geany should select localization basing on Windows UI language (without fiddling with env variables). Now, it doesn't. So there something was broken along the way, and should be fixed.