About [[MediaWiki:Passwordtoolong/es]]
What is the use case for single caracter passwords? I know this has nothing to do with translation, but the message could be simpler deleting PLURAL. And it would be a good security practice not even considering using such passwords.
"mw:i18n#Be aware of PLURAL use on all numbers" may give some indications that PLURAL is not about "is 1" or "is more than 1" :-)