It seems that passing the ":include_default_locale => false" parameter in routes.rb has no effect (ie. include_default_locale is always true).
In routes.rb, what does not work :
filter :locale, :include_default_locale => false
(maybe it is not the way it should be done)
Workaround (set the attribute directly):
RoutingFilter::Locale.include_default_locale = false
So, maybe the problem is : how to set the include_default_locale attribute.
Set it in an initializer, thats how I do it.