When trying to link https://commons.wikimedia.org/wiki/Category:Lactalis with the 'Add Links' menu item on the side menu I chose in the pop-up window as language en and then i put in the field for page 'Category:Lactalis' and when clicking then on the button 'Link page' i got the following error message:
A page "Category:Lactalis" could not be found on "mediawikiwiki".
The external client site "mediawikiwiki" did not provide page information for page "Category:Lactalis".
An this despite the fact that https://en.wikipedia.org/wiki/Category:Lactalis exists.
It is very clear that the fact that the erorr message says "A page "Category:Lactalis" could not be found on "mediawikiwiki"." indicates that this function looks for a wikimedia project 'mediawikiwiki' which obviously does not exist. unfortunately I have no clue to look where this setting was put erronously as I suppose that in some setins or eventually translations en was replaced with mediawiki.