‹hidden-categories› is documented to have one parameter (category count) – see documentation, but is given no parameter under Timeless

As an example, you can consult fr:Cañete (Chili) with the interface in French (note that it will not work in English because of different pluralization rules) and hidden categories shown (Preferences/Appearance/Advanced).

You should see “Catégories cachées” (last section in right column) because the article has more than one hidden category, but “Catégorie cachée” is displayed instead (the singular form).