HtmlAgilityPack set node InnerText

Try code below. It select all nodes without children and filtered out script nodes. Maybe you need to add some additional filtering. In addition to your XPath expression this one also looking for leaf nodes and filter out text content of <script> tags.

var nodes = doc.DocumentNode.SelectNodes("//body//text()[(normalize-space(.) != '') and not(parent::script) and not(*)]");
foreach (HtmlNode htmlNode in nodes)
{
    htmlNode.ParentNode.ReplaceChild(HtmlTextNode.CreateNode(htmlNode.InnerText + "_translated"), htmlNode);
}