Nice idea. It would mean that you would have a spy that was extremly good at spying on eg. Aedui, but if he was to spy on another faction, eg. Romani, you would get a penelty for spying on a faction that you are lacking in info about. It would make spying a lot more complex. Also, you could maybe have a trait called "Interested in culture" or something like that, which would reduce the penalty from spying on a faction that you aren't used to, because you at least know a bit of info about the factions.