Load site default language ID into session unless the one in session …

[dbwinger]

…is associated with the current site.

What is this pull request for?

If at least one site has multiple languages, after the language is switched in the admin/pages view, the language ID is saved to session, and switching to another site does not change the language in session, which causes that language's pages to continue to be shown.

Closes #2956

This should be backported at least to 7.1.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.02%. Comparing base (72973ad) to head (e274774). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2957 +/- ## ========================================== + Coverage 96.00% 96.02% +0.01% ========================================== Files 232 233 +1 Lines 6282 6312 +30 ========================================== + Hits 6031 6061 +30 Misses 251 251 ```

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