Hello everyone,
This is the deal:
Create three articles in english and assign them to two categories like "Cat1-English", "Cat2-English".
Then Create three articles in other language, for example Spanish.
One of them put it inside a category already exist for english articles and the other two in new categories "Cat3-Spanish" & "Cat4-Spanish".
Well, the bug is that whenever you are browsing the site in english or spanish, categories will show all categories, that's: Cat1-English, Cat2-English, Cat3-Spanish, Cat4-Spanish. So if browsing in english you click over a category that do not have articles for that language, the category link which show all articles for a given category will return a Wordpress custom 404 error page.
My two cents :)