I think I was understanding your question. If you follow the guide I linked to, and then load your site with the mobile view by appending ?mobile_view=1 to the URL in your browser’s address bar, you will see something like this when you open the registration modal:
Then look at your web inspector’s console tab and search for Translation #60. It will show that the key used is user.email.title. Then go to your Admin / Customize / Text page and enter the translation key (user.email.title) into the search field. The correct field to edit will show up.
Make sure to not include any spaces at the beginning of the translation key if you are copy and pasting it into the search box.