(Bug 30364) LanguageConverter should depend on the page content language instead...

(Bug 30364) LanguageConverter should depend on the page content language instead of the wiki content language. Remains largely the same, except for extensions using the PageContentLanguage hook like Translate and WikimediaIncubator.
* Made variant tabs hidden on special pages (has no or sometimes wrong/inconsistent effect there; plus it's mainly in the user language)
* Made redirects be in content language again (remove from Title->getPageLanguage())