Use the language of the page, not necessarily the general site

It's possible that the user specificies a generator like
-cat:zh:Category:Blah", in which case the value of
pywikibot.Site().language() may not be "zh", depending
on the user's config. It's also possible that in the future
a generator will yield pages on multiple projects, so this
guards against that as well.

Change-Id: Ic1f10118a2dff93b71861b7ea97f7da22908cb04
Merge "Use the language of the page, not necessarily the general site"