About [[MediaWiki:Templatedata-modal-table-param-importoption-subtitle/en]]
Fragment of a discussion from Translating talk:MediaWiki
There is, in the message before this one, templatedata-modal-table-param-importoption ("Add $1 suggested {{PLURAL:$1|parameter|parameters}}"). That suffices. So PLURAL support on the list length is not really needed here once more.