@JasonJDooley ,
\AppData\Local\Notepad++
Please note, even if you assume the correct prefix to the path, that’s the wrong AppData folder. Notepad++ settings go in %AppData%\Notepad++, which as explained in the FAQ maps to ...\users\username\AppData\Roaming\Notepad++, not to ...AppData\Local\Notepad++ or ...AppData\Notepad++.
But that’s neither here nor there, because Notepad++ grabs the XML from the executable directory’s localization folder, and copies it to the right place in the %AppData% hierarchy.
So, really, the process you described should work.
When I have spanish.xml in the localization\ subdir of my Notepad++ executable directory, and pick Español from the localization dropdown, the UI immediately changes to Spanish; if I look at %AppData%\Notepad++\nativeLang.xml, I can see that it’s picked the Español contents. When I change localization back to English, I see the UI change back; and File > Reload from Disk will refresh the viewed contents of nativeLang.xml, showing them to be English again.
I am wondering if you don’t have write permission to the %AppData%\Notepad++\ directory in general or the %AppData%\Notepad++\nativeLang.xml file specifically.