Inappropriate route

Hello,

In my rails 3 application, I have the following routes.rb :

App::Application.routes.draw do |map|
    scope '/:lang' do
        scope '/:category_id' do
            root :to => 'categories#show', :as => :category
        end
        root :to => 'categories#index'
    end
    resources :categories, :except => [:index, :show]
    root :to => 'main#pick_locale'
end

The rake routes method generates the urls as I'd expect them to be :

/:lang/:category_id            {:controller=>"categories", :action=>"show"}
/:lang                         {:controller=>"categories", :action=>"index"}
categories POST   /categories(.:format)          {:controller=>"categories", :action=>"create"}
new_category GET    /categories/new(.:format)      {:controller=>"categories", :action=>"new"}
             PUT    /categories/:id(.:format)      {:controller=>"categories", :action=>"update"}
category DELETE /categories/:id(.:format)      {:controller=>"categories", :action=>"destroy"}
edit_category GET    /categories/:id/edit(.:format) {:controller=>"categories", :action=>"edit"}
root        /                              {:controller=>"main", :action=>"pick_locale"}

However when I use url_to(@category), it generates the following url :

categories/test?lang=en

Instead, of course, of :

en/test

When I go to this url generated, categories#show is being called. If I go to the expected url, the action is called too.

However if I remove the root :to => 'categories#show' call, none of the two urls are valid anymore.