Fallback to $LANG if $LC_ALL isn't set.

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=15214

This is a regression introduced in be532638.

Xamarin.Android assumes that the $LANG environment variable can be
used to control System.Globalization.CultureInfo.CurrentCulture,
setting LANG based on the current Java locale.

This was broken in be532638 because $LANG was no longer being checked
to determine the default locale, as $LC_ALL was not set.

Fix the fallback logic so that $LANG is checked when $LC_ALL is not.