Hello 
This theme looks like to me Sam’s Simple Theme? Which override the template and adds theme translations where you can translate these.
Go to: /admin/customize/themes/
Select: Sam's Simple Theme
This page you will see the theme translations.
Of course, It’s only works if I was correct and you use this theme. 
If I was wrong with this - I am pretty sure you will find these translations in a theme or theme component you use because in Discourse core this section is different.