Fix not-loaded DbPageLanguage when Title::getPageLanguage() get's called

Fix not-loaded DbPageLanguage when Title::getPageLanguage() get's called

If the Title object isn't the title of the current viewed WikiPage, the page_lang
field of the database isn't requested. This results in the problem, that
Title::getPageLanguage() always returns the default content language, even if
the page language is different (changed with Special:PageLanguage, if
wgPageLanguageUseDB is true). That is problematic for the Translate extension,
which relies on the correct page language.

This change makes sure, that getPageLanguage() always return the correct page
language. If the page language isn't loaded already, Title::getPageLanguage()
now does a database lookup (if $wgPageLanguageUseDB is true) to get the correct
page language. It will use LinkCache for the page_lang field.

Bug: T121666
Change-Id: I0ae5ea39f7a124ed427ca5dfb26c1a116b27a94e