I sent you a PM message.
We've checked how the email template editing works.
If you are within the email template form, the restore button/link will only work if you haven't saved the form.
Otherwise, you'll have to use the links to restore defaults outside of the email template editing form, which should restore all email templates (including English, which may override any changes you've made...).
Also, have you tried clearing your dataase?This post
is a bit old, I think, but you can try to clear your database data and restore again to see if it does the trick.
I'm not sure if they implemented the script mentioned in a post above
the one I linked to.