org.zkoss.zk.ui.sys.HtmlPageRenders.outDivTemplateBegin() outputs a non-localized loading indicator ("Processing...") when initially loading the page:

out.write(" class=\"z-temp\"><div id=\"zk_proc\" class=\"z-loading\"><div class=\"z-loading-indicator\"><span class=\"z-loading-icon\"></span>Processing...</div></div>");

This issue might be related to ZK-1505.

Furthermore, the z-temp class adds a mask that gets duplicated and as such significantly darkens the entire page for a brief period (see attached screenshots).