Nice extension! You may want to drop some parts of your code such as tests and the Admin JS since those aren't really being used 🙂 you should also remove any irrelevant resources such as the admin.less file and remove both the admin.js and admin.less code from your extender file. I would also suggest referencing core language strings that already exist in core itsel that you can reuse.
If you'd like, I could submit a PR to help you clean up a bit?
