I'm sorry but I don't understand what you are talking about. How is this connected to mails? I don't have mails folder in my themes folder.
My issue is that after upgrade several strings not provided by Classic theme stopped showing up translated on the front, even though they have visible translations in the backoffice. Following this guide I was able to make some strings work again, but those that don't show up directly in template files still get displayed on the front in default language.