* CategoryPage.php: fix the "category-empty" message per comment on r91518, it was...

* CategoryPage.php: fix the "category-empty" message per comment on r91518, it was outside the bodyContent. And do not convert interface text.
* Parser.php: Apparently that was a much bigger bug: do not convert interface text going through the parser either.
* Preferences.php: Do not convert the user signature.
For this bug in action, see e.g. http://sr.wikipedia.org/sr-ec/Посебно:Подешавањ?uselang=en (e.g. "Username" -> "Усернаме")