GWT UI-Binder I18N

Question

I want to add Internationalization to my GWT Project. I am using GWT 2.10. It is not working ...

My Ui-Binder code is ..

<ui:UiBinder xmlns:ui="urn:ui:com.google.gwt.uibinder"
xmlns:g="urn:import:com.google.gwt.user.client.ui"
xmlns:b="urn:import:org.gwtbootstrap3.client.ui"
xmlns:b.gwt="urn:import:org.gwtbootstrap3.client.ui.gwt"
xmlns:b.html="urn:import:org.gwtbootstrap3.client.ui.html"
xmlns:b3="urn:import:org.gwtbootstrap3.extras.toggleswitch.client.ui"
ui:baseMessagesInterface="com.lass.smsweb.client.SiMaSoMessages"    
>

In the file itself ...

        <b.html:Span><ui:msg description="ST1">Fehler</ui:msg></b.html:Span>

Then the interface file names SiMaSoMessages.java. (Note, I did not add the interface stub for ST1 as I have read, that it is not necessary. I even tried, still not working)

package com.lass.smsweb.client;
import com.google.gwt.i18n.client.Messages;
public interface SiMaSoMessages extends Messages {
  @DefaultMessage("''{0}'' ist keine gültiges Symbol.")
  String invalidSymbol(String symbol);  
} 

and now the properties file SiMaSoMessages_fr.properties in package com.lass.smsweb.client

invalidSymbol = ''{0}'' ne pas un valid numero.
ST1 = Erreur
ST2 = Perturbations
ST3 = Alarmes
ST4 = Maintenance 

What am I doing wrong here ...