Test:
Create a multilanguage site (2 languages are enough).
Make sure you have 1 category and 1 subcategory per language (test with com_content for example)
Create a List All Categories menu item with base set to ROOT in each language.
Display the menu item in frontend: the categories not belonging to the current language will be displayed.
Their article count is 0 and clicking on the + icon has no effect.
Clicking on their title will fail to display the category as it keeps the wrong URL Language Code which is the one of the Current language.
In the screenshot below the link to "English Categories" is /index.php?option=com_content&view=category&id=10&Itemid=622&lang=fr as we are in the French interface.
Result:
Patch and test again:, you will get.
This patch also normalises the urls in default_items as done elsewhere in core. It has no impact on the results when the categories are filtered.
zero-24
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Test : patch works fine on last staging. Thanks!