I tried this myself just now by adding and removing the "slovenly speech" personality from an NPC, and for adding I got the message that the NPC already had that trait, then for removing that the NPC doesn't have the trait. It was still added and then removed successfully. Seems to be a bug in the action description.
Does the NPC still speak that way? If so, click on the character's name in the list on the right, then check the "Personality" section - what does it say?