Changing the site default language shuld not be possible if Canvas pages exist

Canvas modifies the content translation settings form to:

• not allow to set a different Canvas entity language other than the site default language
• not allow to expose a language selector on Canvas entities

What Canvas does not do is actually lock down changing the site default language. So sites can get into a situation where they change the site default language and now all assumptions in Canvas about what the config and content language should be are broken. Even if config language follows the site default language change (such as using Config Language Lock module), the content language will still mismatch, which leads to WSODs such as #3591850. Also it leads to usability issues where the language selector in Canvas lies to the user: #3591793 (closed)

There also adjacent issues in related modules such as Canvas Translate Extension. https://www.drupal.org/project/canvas_translate/issues/3616274 provides a full resolution to that with updates specs and tests already, which could be committed without changing anything in Canvas. It just needs to land :)

If those issues are not to be resolved in Canvas, Canvas should disallow changing the site default language.