Hi rolo
Thanks for reaching out
To make that change without a plugin, you could use a child theme. Make a copy of single.php
and insert it into the child theme and then translate the strings in that file.
If that’s how you’d like to proceed, here is a child for Polestar: https://purothemes.com/documentation/polestar-wordpress-theme/child-theme-library/#heading-starter and here is how to migrate to a child theme if you aren’t using one already: https://purothemes.com/customizer-settings-export-import/