Wo änder ich diesen Titel? Das letzte Kommentar muss genehmigt werden.

in the lang files of formal : de_DE_formal.po it is correct : “Der letzte Kommentar muss genehmigt werden.”
( i.e. when using the formal form of address “SIE” )

and indeed on the informal de_DE.po it is incorrect: “Das letzte Kommentar muss genehmigt werden.”

So you can either switch to the formal Version of the lang file ( on dashboard – settings – site language )

Or you can edit that lang file ( using poedit ) and have your own child-theme langfile for that:
(edited Versions of both files .po and .mo files to upload in subfolder of your child-theme: lang )

function overwrite_language_file_child_theme() {
    $lang = get_stylesheet_directory().'/lang';
    return $lang;
}
add_filter('ava_theme_textdomain_path', 'overwrite_language_file_child_theme');

or – quick and relative dirty way:
( in child-theme functions.php )

function my_text_strings( $translated_text, $text, $domain ){
  switch ( $translated_text ){
    case 'The last comment needs to be approved.' : $translated_text = __( 'Der letzte Kommentar muss genehmigt werden.', $domain ); break;
    // case 'Das letzte Kommentar muss genehmigt werden.' : $translated_text = __( 'Der letzte Kommentar muss genehmigt werden.', $domain ); break;
  }
  return $translated_text;
}
add_filter('gettext', 'my_text_strings', 20, 3);

( if first case does not work uncomment the second case in that code and remove the first case )

or use a plugin to correct that wrong translation