Steps to Reproduce

<?import org.zkoss.util.Locales?>
<zk>
	<zscript>session.setMaxInactiveInterval(5);</zscript>
	<label onCreate="self.setValue(Locales.getCurrent().toString());"/>
	<button label="click after 5 seconds" onClick=""/>
</zk>

access the page using a locale request header different to the server's default

click the button after >5sec to cause a session timeout error

Current Result

the error text is always shown in the default language of the server
the dialog buttons (reload / cancel) are localized as expected

Expected Result

the localized message as provided for different languages should be used
e.g.:
english label (msgzk.properties)
german label (msgzk_de.properties)

Debug Info

in order to test you can change your browser's request locale to a different than the server's default locale

Root Cause

Default Ajax request handling between Charsets.setup() and cleanup():
https://github.com/zkoss/zk/blob/v8.5.0/zk/src/org/zkoss/zk/au/http/DHtmlUpdateServlet.java#L452-L460
Session timeout message outside the block above:
https://github.com/zkoss/zk/blob/v8.5.0/zk/src/org/zkoss/zk/au/http/DHtmlUpdateServlet.java#L470