The problem is that the plural rules are defined in two places. The other one was recently changed to use a library (DEV: Upgrade the MessageFormat library (JS) · discourse/discourse@301713e · GitHub).
However, the plurals defined in the library are not always the same as the ones Discourse used before, and the ones in plurals.rb weren’t changed. Inconsistency in plural definition
But that does not explain why Crowdin created a separate version for many