So it seems the best solution is reversing things: When I let RentalDescription have a “has one” relationship to Rental, then fetch RentalDescriptions and populate the associated Rental, then I more or less get the data I need in the correct locale. It’s not pretty, but it works.
Thanks @Lucas_Ramos for providing the solution to a similar problem you had a year ago (Relation between localized and not localized content type)