What is the result of GENDER in case of plurality ?
Yup, I know, I just wanted to point out that it's really just one of those cases that is problematic, and not two as you said in the post. :-)
This data is available in Unicode CLDR as far as I know: https://github.com/unicode-cldr/cldr-core/blob/master/supplemental/gender.json
I think it would be relatively simple to implement it in MediaWiki. Would someone like to file a task in Phabricator and CC me?