See the part about adding an environment file on Windows. On Linux, I set my LANG to en_US.UTF-8 but all the LC_* variables (apart from LC_ALL) to en_CA.UTF-8 and that gets me the right UI/formats mix in GnuCash (and also Chromium, which tries to speak British to me otherwise), and hopefully that works on Windows too.
See the part about adding an environment file on Windows. On Linux, I set my LANG to en_US.UTF-8 but all the LC_* variables (apart from LC_ALL) to en_CA.UTF-8 and that gets me the right UI/formats mix in GnuCash (and also Chromium, which tries to speak British to me otherwise), and hopefully that works on Windows too.