Currently unused, but planned to provide support for marking individual
language links in the UI, e.g. for featured articles.
+'LanguageSelector': Hook to change the language selector available on a page.
+$out: The output page.
+$cssClassName: CSS class name of the language selector.
+
'LinkBegin': Used when generating internal and interwiki links in
Linker::link(), before processing starts. Return false to skip default
processing and return $ret. See documentation for Linker::link() for details on