There is no specific location for any localisation text; it goes in whatever localisation file is most convenient (Paradox normally does a single file for each expansion or patch that adds new text).Btw, do you know where the localization text for modifiers is? I was going to add a new modifier first but couldn't find where to put the descriptions. I see various files for events and other things but nothing for modifiers.
Any new text you add would ideally be in a new file.
Just search all the localisation files for united_japan to see what the format looks like.