TYPO3 Core

The current link to that documentation page is

https://docs.typo3.org/typo3cms/extensions/core/Changelog/7.4/Feature-33071-AddTheHttpHeaderContent-LanguageWhenRenderingAPage.html

The patch in question is https://review.typo3.org/#/c/40265/

    /**
     * Site content selection uid (can be different from sys_language_uid if content
     * is to be selected from a fall-back language. Depends on sys_language_mode)
     * @var int
     */
    public $sys_language_content = 0;

sys_language_content is a term/property defined in TypoScriptFrontendController (TSFE) which, as the PHPDoc above states, can differ from sys_language_uid.
Nonetheless the term is irritating in the feature description, it should be updated, file in question is /typo3/sysext/core/Documentation/Changelog/7.4/Feature-33071-AddTheHttpHeaderContent-LanguageWhenRenderingAPage.rst in the Branch TYPO3_7-6

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/55410

Applied in changeset 950219d560095273b91fa69014853a5a11c62613.