Changing LC_MONETARY does not affect number format

[Mario G]

I've installed the latest version of gnucash on my computer. I need to 
have "." as a decimal separator because I'd like to use the keypad to 
enter numbers, without having to switch to the other portion of the 
keyboard to enter the comma.
Setting LC_MONETARY=en_US in the environment file (I'm using Windows 
8.1) does not change anything: numbers are displayed like "1.055,23", 
"0,15" while I'd like them to be displayed as "1 055.23" and "0.15".

I have this in the environment file:
# If you wish GnuCash to use a different language, uncomment the two 
parameters
# below and set LANG to your preferred locale
LANG=en_US
LANGUAGE={LANG}

If I change LANG=en_US to LANG=it_IT I have italian language for the 
interface and the same number format.
If I put LC_MONETARY=en_US after the lines shown above, nothing changes. 
I also tried LC_MONETARY=de_CH without result.

Can you help me?

Thanks
Mario