I issued a JIRA:

https://issues.jboss.org/browse/SEAMINTL-72

I implemented a surrogate of org.jboss.seam.international.status.Messages as a temporary workaround.

package org.mydomain.myapp.facesutils;

import static javax.faces.application.FacesMessage.SEVERITY_ERROR;
import static javax.faces.application.FacesMessage.SEVERITY_FATAL;
import static javax.faces.application.FacesMessage.SEVERITY_INFO;
import static javax.faces.application.FacesMessage.SEVERITY_WARN;

import javax.enterprise.context.RequestScoped;
import javax.faces.application.FacesMessage;
import javax.faces.context.FacesContext;
import javax.inject.Named;

/**
 * Since Messages in Seam 3.1.0.Beta is BUGGED, I made mine. 
 * @author luca
 *
 */

@Named
@RequestScoped
public class Messages {

     public void info(String summary) {
          FacesContext cxt = FacesContext.getCurrentInstance();
          FacesMessage fm = new FacesMessage();
          fm.setSummary(summary);
          fm.setSeverity(SEVERITY_INFO);
          cxt.addMessage(null, fm);
     }
     
     public void info(String summary, String detail) {
          FacesContext cxt = FacesContext.getCurrentInstance();
          FacesMessage fm = new FacesMessage();
          fm.setSummary(summary);
          fm.setDetail(detail);
          fm.setSeverity(SEVERITY_INFO);
          cxt.addMessage(null, fm);
     }
     
     public void warn(String summary) {
          FacesContext cxt = FacesContext.getCurrentInstance();
          FacesMessage fm = new FacesMessage();
          fm.setSummary(summary);
          fm.setSeverity(SEVERITY_WARN);
          cxt.addMessage(null, fm);
     }
     
     public void warn(String summary, String detail) {
          FacesContext cxt = FacesContext.getCurrentInstance();
          FacesMessage fm = new FacesMessage();
          fm.setSummary(summary);
          fm.setDetail(detail);
          fm.setSeverity(SEVERITY_WARN);
          cxt.addMessage(null, fm);
     }
     
     public void error(String summary) {
          FacesContext cxt = FacesContext.getCurrentInstance();
          FacesMessage fm = new FacesMessage();
          fm.setSummary(summary);
          fm.setSeverity(SEVERITY_ERROR);
          cxt.addMessage(null, fm);
     }
     
     public void error(String summary, String detail) {
          FacesContext cxt = FacesContext.getCurrentInstance();
          FacesMessage fm = new FacesMessage();
          fm.setSummary(summary);
          fm.setDetail(detail);
          fm.setSeverity(SEVERITY_ERROR);
          cxt.addMessage(null, fm);
     }
     
     public void fatal(String summary) {
          FacesContext cxt = FacesContext.getCurrentInstance();
          FacesMessage fm = new FacesMessage();
          fm.setSummary(summary);
          fm.setSeverity(SEVERITY_FATAL);
          cxt.addMessage(null, fm);
     }
     
     public void fatal(String summary, String detail) {
          FacesContext cxt = FacesContext.getCurrentInstance();
          FacesMessage fm = new FacesMessage();
          fm.setSummary(summary);
          fm.setDetail(detail);
          fm.setSeverity(SEVERITY_FATAL);
          cxt.addMessage(null, fm);
     }

}

This doesn't sound like a problem with the International module, as it doesn't control anything to do with how the messages are displayed in the View layer.

It's more likely a problem with Faces.

Thanks, Jason! It was a problem wirt seam-faces. As a matter of fact, after I added the following dependencies, messages started being displayed.
Strange how no ClassNotFoundException was being thrown, though...

<dependency>
     <groupId>org.jboss.seam.faces</groupId>
     <artifactId>seam-faces</artifactId>
</dependency>
<dependency>
     <groupId>com.ocpsoft</groupId>
     <artifactId>prettyfaces-jsf2</artifactId>
</dependency>

However, now I get the FacesMessage(s) have been enqueued, but may not have been displayed warning for every message I add, despite there's a <h:messages> in the page.