PLURAL keyword for languages without grammatical plural forms
Correct me if I'm wrong but this is only an issue for messages with special n=0 cases (for which we usually use different messages altogether).
I can see the short term fix will work most of the case. Sometimes we'd have to choose translation a bit inconsistent to original, but the difference would be minor anyway.
I'm not sure how to check whether it's a 'n=0' case or not. If MediaWiki:badaccess-groups/ja is a non 'n=0' case, I confirm it's also affected.