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.
So e.g. "english" is not always just "en", but "en_UK", "en_US" etc. This should fix issues similar to bug #690535. Used http://en.wikipedia.org/wiki/ISO_3166-1 as reference.