[Risolto] Admin's wordpress user interface language keeps changing

Background of the issue:
I have configured the admin language to English on my WordPress user profile to ensure that my user interface remains in English when editing pages in other languages. This setup used to work correctly. The issue can be seen at link nascosto.

Symptoms:
Even though the admin language is set to English, the user interface changes to French when I visit a French page or the list of French pages.
Viewing the French pages literally changes my user profile language to French, even though it shouldn't. Even when I change the language in the top navigation to English, instead of just changing the UI language to English, it navigates me away from the French list of pages and back to the English ones! And there is no way to edit French pages with an English user interface.

Questions:
Why does the admin interface language change to French when visiting French pages?
How can I ensure the admin interface remains in English regardless of the page language?