Have since a small module (wordpress) that does not automatically more language can. Since WPML finds no string.
$html .= '<li class="weiterlesen"><a href="' . esc_url( get_permalink() ) . '" class="">' . $args['readmore_text'] . '</a></li></ul></div>';
replace . $args['readmore_text'] . to <?php _e( 'readmore', 'tpl' ); ?>
Try below code
$html .= '<li class="weiterlesen"><a href="' . esc_url( get_permalink() ) . '" class="">' . __( 'readmore', 'tpl' ) . '</a></li></ul></div>';