@bakagaijin
don't worry - just keep it as marquis in every version.
historically and story-wise, it'd make more sense for the Randolph House to be a dukedom. but japanese isekai stories have a very poor grasp of medieval europe history/customs, since they use their own incomplete understanding of japanese medieval custom as a basis (cue prevalence of harems unthinkable in christian europe, unbound behaviour by nobles and church officials, nationalism vs feodalism, professional standing armies in the thousands ...)
if the author uses marquisat, then marquisat it has to be in the translation - no sweat.