For polls of type range voting (histogram range) with comment, the text [comments] is shown above the text boxes where one can provide comments. This implies that it is customisable per election, but it is not, nor is it translateable through Special:SecurePoll/translate (as other similar fields, like [intro] and [return-text], are).
As suggested by @tstarling here, it may make more sense for this to be a MediaWiki message than one which alters per election.