$manufacturer = new Manufacturer($product['id_manufacturer'], $id_lang);
$category = new Category($product['id_category'], $id_lang);
$items_table .=
и
<td style="padding:0.6em 0.4em;"><strong>'.$manufacturer->name.'</strong></td>
<td style="padding:0.6em 0.4em;"><strong>'.$product['category_name'].'</strong></td>
или
<td style="padding:0.6em 0.4em;">'.$category->name.'</td>