Hi!
This message is used as title of Special:Categories where second form must be used. Belarusian/Russain plural function without arguments will return 3 incorrect argument: Катэгорый/Категорий.
You can translate the message as {{PLURAL:$1|Катэгорыя|Катэгорыі|Катэгорыі}}
As far as I understood in context of Special:Categories title it's not a case about a number of objects (where PLURAL could be used in generic case). It's case about one and many objects. By the chance it works as intended for English language PLURAL, but will fail in Belarusian/Russian language case (and probably other cases).
So PLURAL function should not be used when number is not mentioned (generic one object/many objects case).
Eugene.