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}}
This solution will work correct while number of categories <= 20
--
Alexander Sigachov