[Закрытые] subdirectory of my website is showing source language content (it worked fine yesterday and the othe...

Background of the issue:
I need the Spanish version of my site to show up in Spanish as it used to yesterday. Link to a page where the issue can be seen: скрытая ссылка

Symptoms:
The "destination" pages in the /es/ subdirectory of my website are showing the source language content. It worked fine yesterday, the problem doesn't seem related to plugins or wp updates, and the other languages are working fine at the moment. Instead of Spanish, on the /es/ destination pages, I got English.

Questions:
Why is the Spanish version of my site showing English content?
What can I do to fix the language display issue in the subdirectory?...