Flush rewrite rules when language changed or translation updated

[merkushin]

Resolves #7432

Proposed Changes

• Flush rewrite rules when the language was changed or the translation was updated.

Testing Instructions

0. Let's assume your current Site Language is English (US) and you don't have other languages "installed".
1. Create a course, publish it.
2. Open it on the frontend following the link in the sidebar. Make sure it opened successfully.
3. Go to General Settings and change Site Language to German.
4. Go to Dashboard > Updates and update the translation.
5. Go to the course, open the course on the frontend following the link in the sidebar. Make sure it opened without 404 error.
6. Go to General Settings, change the language back to English (US). That means you don't need to update the translation.
7. Go to the course, open the course on the frontend following the link in the sidebar. Make sure it opened without 404 error.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Legacy courses (course without blocks) are tested
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (406e9ed) 51.20% compared to head (52451a2) 51.20%.
Report is 1 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk    #7435   +/-   ##
=========================================
  Coverage     51.20%   51.20%           
- Complexity    11184    11188    +4     
=========================================
  Files           614      614           
  Lines         47220    47228    +8     
  Branches        405      405           
=========================================
+ Hits          24179    24185    +6     
- Misses        22714    22716    +2     
  Partials        327      327           

Files	Coverage Δ
includes/class-sensei.php	25.26% <75.00%> (+0.87%)	⬆️

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[Imran92]

Works as described 👍 I remember during the janitorials, we used to suggest users to go to Settings -> Permalinks and just click Save Changes for fixing issues like this. Doing it automatically is a nice step!