i18n: detect UI language without POSIX-style locale variable on Windows (BC)
On Windows, it isn't common to set LANG environment variable. This patch makes
gettext honor Windows-style UI language [1] if no locale variables are set.
Because of this change, LANG=C or HGPLAIN must be set in order to disable
translation on non-English Windows.
[1]: http://msdn.microsoft.com/en-us/library/dd374098(v=VS.85).aspx
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html