The singular form works correctly. But the plural doesn’t, it’s not translated, it remains “categories”. And this happens only with this particular table. Other tables plurals are translated correctly. It feels like a bug caused by the italian translation of this plural word being exactly a prefix without the -s of the original word.
makemessages sometimes generates translation entries marked as fuzzy, e.g. when translations are inferred from previously translated strings. By default, fuzzy entries are not processed by compilemessages.