It could be you are referring to these prior discussions, but in case not:
In summary: we’ve never felt like this was a bad idea, but it is impossible for all practical purposes for us to implement it. You can’t tell the Mac to only highlight some kinds of words in this text window and other words in this other split over here and a third dictionary for the Notes in the Inspector—or even in a broader sense to say that one project uses UK English and another French. The same highlighting that is done while you type into Safari or Mail is what you’re seeing in Scrivener.