The vulnerability stems from improper handling of the '_com_liferay_portal_language_override_web_internal_portlet_PLOPortlet_key' parameter. 1) The PLOPortlet's render method likely retrieves and passes the raw parameter value to the view layer without sanitization. 2) The JSP view then directly embeds this unescaped value into the HTML response. This two-step failure (lack of input validation in controller, lack of output encoding in view) creates the XSS vector. The confidence is high due to the specific parameter name matching Liferay's portlet namespace conventions and the reflected XSS pattern requiring both controller and view layer vulnerabilities.