How to translate the author line "Submitted by ... on ..."
While the page /admin/config/regional/content-language holds the actual translated text, it's not the place where the &quot;Submitted by&quot; translation is derived from. To do so you need to

Navigate to Configuration &gt; Regional &amp; language &gt; User interface translation
Search for &quot;Submitted&quot; in the String contains field. Note the uppercase S (thank you @Hudri).
Now enter the translation and save it. Also note the placeholders for author and date information.