How to change the language of strings like "read more" and "submitted by"?
Install the core modules Language and Interface Translation 
Add other language (admin/config/regional/language)
Configure the Selected language setting (admin/config/regional/language/detection)
To have a different language in admin enable the detection method Account administration pages
Go to user/1/edit and in Language settings section select your Administration pages language
Translate your strings (admin/config/regional/translate)