Manual:Hooks/PageContentLanguage

Details

 * $title: the Title object
 * &$pageLang: Language object or string; the page content language, either pass a Language object or a string (language code)
 * $language: Language or StubUserLang object; the user interface language

It was added in Title.php but is in content/ContentHandler.php since.

Example
For example, if you want to mark all pages in the namespace with index 200 (and its talk page 201) as being in French, use: