This thread is resolved. Here is a description of the problem and solution.
Problem: 404 Error - Posts or Pages are not found in second language.
Solution: A 404 errors in most cases can be solved by going to Settings -> Permalinks and simply applying the "Save" button.
Relevant Documentation: https://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-posts-returning-404-error/
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