Language attribute is not attached to node title on a full page view

Problem/Motivation

#1164926: Nodes need to have languages specified separately for accessibility → points out that since you can view the content of a node in one language while browsing the site in a different language, to comply with to WGAG 3.1.2 Language of Parts, we need to add lang attributes to content. This was done in #1164926: Nodes need to have languages specified separately for accessibility → but the node title in a full page view was neglected. (Pointed out in #1164926-87: Nodes need to have languages specified separately for accessibility → .)

This issue has been added to 📌 [meta] Conform to WCAG Success Criterion 3.1.2: Language of Parts Active .

Steps to reproduce

TBD. We'll eventually need steps to reproduce so folks can test this.

Any solution will need to pass an accessibility test of WGAG 3.1.2 Language of Parts and a review from someone in the Accessibility team.

Proposed resolution

Add lang attributes (with what value?) to the HTML output for page titles for all core theme templates and default HTML output for page titles defined in renderers.

Remaining tasks

All of them remain. This needs to be discussed and tasks determined first.

1. Discuss what changes need to be done and what the implications are. The page title is special and the first thing to do would be to identify all the places where the page title is output, in core theme templates, and also in classes such as Drupal\Core\Render\MainContent\HtmlRenderer (others?)

(Update this list with all the places that need an update.)

User interface changes

TBD.

API changes

TBD.

Data model changes

TBD.

Release notes snippet

TBD.