Posts / Missing translation

Hello,

i have a website and use the “posts” plugin.

Now i have 3 words/strings that are in english, but the website is german and also german is selected in backend. The website is only available in german, so i don′t use wpml/loco translate or a other translations plugin.

The words/strings that acutally in the post plugin are visibible in EN are the follows:
Categories: xxx
Published On: xxx
By xxx

How i can see this in German?

In my child theme (functions.php) for “Published On:” i try this function, but without success:
add_filter( ‘gettext’, ‘meine_beitraege_uebersetzung2’ );
function meine_beitraege_uebersetzung2( $translation2 ) {
if (‘Published On:’ === $translation2 ) {
$translation2 = ‘Ver?ffentlicht:’;}
return $translation2;}

I hope you can help me.

Yours sincerely
Tania