Bug 675554 - fix overriden language environment variables on Windows.

If any of the locale environment variables used for UI language is set
(LANGUAGE, LC_ALL, LC_MESSAGES and LANG), do not let Windows' default UI
language override these.
Language set in GIMP preferences still have higher priority.
(cherry picked from commit fe481275e2e42a937618366d18cd6aa03bec3b59)