因为文本由_() 当然，您可以使用gettext 滤器

function change_update_notification_msg( $translated_text, $untranslated_text, $domain ) 
{

    if ( is_admin() ) {
        $texts = array(
            \'There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a>.\' => \'My custom notification. There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a>.\',
            \'There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a>. <em>Automatic update is unavailable for this theme.</em>\' => \'My custom notification. There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a>. <em>Automatic update is unavailable for this theme.</em>\',
            \'There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a> or <a href="%5$s" %6$s>update now</a>.\' => \'My custom notification. There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a> or <a href="%5$s" %6$s>update now</a>.\'
        );

        if ( array_key_exists( $untranslated_text, $texts ) ) {
            return $texts[$untranslated_text];
        }
    }

    return $translated_text;
}
add_filter( \'gettext\', \'change_update_notification_msg\', 20, 3 );