Hi!
PLURAL is used incorrectly in 'categories' message ('{{PLURAL:$1|Category|Categories}}'). For Belarusian/Russian languages this message should not take in account actual number. Just as in English single and plural form:
For one category: Катэгорыя/Категория For many categories: Катэгорыі/Категории.
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.
I think best solution is to split this message to Category/Categories messages.
With best regards, Eugene.