Unstub $wgLang for PageContentLanguage hook

Unstub $wgLang for PageContentLanguage hook

This is to avoid annoying fatal errors when someone annotated their
hook handler to only expect Language objects, but that expectation
is violated due to this code possibly passing StubUserLang to hooks,
some of which may also assign it to $pageLang.

Even with this in place, it is probably a good idea for hook handlers
to refrain from type hinting parameters that are passed by reference
because their types cannot be guaranteed.

Bug: T214358
Change-Id: I88405a8de4b13675eb5a9d11e9ddc87e20a85fb4