"the library just knows these rules for each language as part of the standard" sounds great until you try to support a small minority language that the library just doesn't know about and then you're left trying to hack around it by pretending that it's actually a regional variety of another language with similar plural rules.

AFAIK, unlike gettext, MessageFormat doesn't allow you to specify a formula for the plural forms as part of the localization data, so the variant selection logic ended up in the hands of library developers rather than localizers or application developers.

And the standard does get updated occasionally, which can also lead to bugs with localization data written against another version of the standard: https://github.com/cakephp/cakephp/issues/18740