There have been a few issues now and then with gettext and friends on Win32.
A few related tracker items are
[ 1250581 ] WIN32: Inkscape uses locale settings for determining UI lang [ 1246076 ] Mixed languages in UI [ 1246463 ] Language Selection Option / Menu Entry
After talking with Peter some last night, it was bubbling around in my brain, and kicked out a work-around.
This will need to be tested/refined, but the basic approach could be to hook into things right at program startup (over in the Win32 bits). At that point we could check if LANG and/or the other POSIX environment variables were set. If not, we could then just query the Win32 NLS calls and then build up and set LANG and LC_* properly.
Some preferences could be set to affect this, and perhaps override it with explicit user values/choices.
The details need to be hammered out, but I think this is a good work- around to allow us to get a better user experience on Win32 without having to get libc changed, etc. I wanted to toss this out in case others were still pondering things today.