i18n: Simplify localization

Now we use gettext just as it is meant to be used. According to [1] and
[2] this could also make l10n work on windows, I'll test that later.

In the test in the set_lang() method we now write all the relevant
environment variables, before we did this there where distribution
specific differences in the behaviour.

[1] http://www.vultaire.net/blog/2010/12/23/i18n-with-gettext-and-python-on-windows-7/
[2] http://stackoverflow.com/questions/3678174/python-gettext-doesnt-load-translations-on-windows