Somehow, your langs.xml has errors in it. Perhaps while exploring various Notepad++ options you accidentally made unintended changes to it. That's what happened to me.
Go to the installation folder for Notepad++ and rename langs.xml to langs.xml.bad. Then, in that same folder, find langs.model.xml, make a copy of it, and rename the copy to langs.xml.
This fixed this same problem for me, hopefully it will work for you as well. You will want to then compare the langs.xml to langs.xml.bad and see if there is anything legitimate that you want back.