Follow up information.

Tried to add another language just to see if I could translate anything - added Swedish
Same error occurs.

Seems a conflict between already pre-packed languages within the Plugin, and languages added after/manually via plugin.

What am I missing here? How can I resolve this?

Maybe release 6.1 from https://github.com/ColdTrick/translation_editor/releases works better. As I don't use the plugin I can't say. As for working with existing language files in plugins it might just be necessary to place the Translation Editor plugin below the other plugins you want to change the translation of.