I'm not sure this is something that you can easily work around...by decorating the TranslationCategory with the ValidatorAttribute you're essentially telling MVC "please automatically validate all instances of this type" - I'm not sure if MVC has the ability to exclude certain properties from validation in certain scenarios...I don't think this is something it supports out of the box, so you'd probably have to write a custom implementation of System.Web.Mvc.ModelValidatorProvider for that.
Jeremy