$dom->loadHTML('<?xml encoding="UTF-8">'.$html);

$contentType = '<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">';
$html = preg_replace('(<meta charset="utf-8">)', '<meta charset="utf-8">' . $contentType, $html)
$dom->loadHTML($html);

eccube.event.render.admin_product_category_edit.before:
    - [onRenderAdminProductCategoryEditBefore, NORMAL]
eccube.event.controller.admin_product_category_edit.after:
    - [onAdminProductCategoryEditAfter, NORMAL]
eccube.event.render.product_list.before:
    - [onRenderProductListBefore, NORMAL]

// $response->setContent($dom->saveHTML());
$newHtml = html_entity_decode($dom->saveHTML(), ENT_NOQUOTES, 'UTF-8');
$response->setContent($newHtml);