Сложно сказать что-либо, потому что нужно смотреть по-живому
var_dump ($data['product_description']);
foreach ($data['product_description'] as $language_id => $value) {
$this->db->query("INSERT INTO " . DB_PREFIX . "product_description SET product_id = '" . (int)$product_id . "', language_id = '" . (int)$language_id . "', name = '" . $this->db->escape($value['name']) . "', meta_keyword = '" . $this->db->escape($value['meta_keyword']) . "', meta_description = '" . $this->db->escape($value['meta_description']) . "', description = '" . $this->db->escape($value['description']) . "', tag = '" . $this->db->escape($value['tag']) . "', seo_title = '" . $this->db->escape($value['seo_title']) . "', seo_h1 = '" . $this->db->escape($value['seo_h1']) . "'");
}