Getting troubles with routes and locales, getting an exception on one route, sound strange...

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
  before_filter :set_locale

  def set_locale
    I18n.locale = params[:locale] || I18n.default_locale
    @language = I18n.locale
  end
end

application_white.html.erb

# other stuffs before
     <div id="transition-bottom" class="transition">
        <div class="inner-transition c100">
          <ul>
            <li class="films"><%= link_to t(:all_movies), films_path %></li> # http://localhost:3000/fr/films
            <li class="panorama"><%= link_to t(:panorama), panorama_path %></li> # http://localhost:3000/fr/panorama
            <li class="small"><%= link_to t(:movie_proposition), propositions_path %></li> # http://localhost:3000/propositions <= Missign /fr here (and only here !)
            <li class="qui small"><%= link_to t(:who_are_we), about_us_path %></li> # http://localhost:3000/fr/about_us
            <li class="utilisation small"><%= link_to t(:cgu), utilisation_path %></li>  # http://localhost:3000/fr/utilisation
            <li class="mentions small"><%= link_to t(:mentions), mentions_path %></li> # http://localhost:3000/fr/mentions
          </ul>
        </div>
      </div>
# other stuffs after

propositions_controller.rb

class PropositionsController < ApplicationController
  layout 'application_white'

  def index
    @proposition = Proposition.new
    @proposition.files.build
  end

  # POST /propositions
  # POST /propositions.json
  def create
     @proposition = Proposition.new(params[:proposition])

     respond_to do |format|
       if @proposition.save
         logger.info { "proposition is valid" }
         format.html { redirect_to :index, notice: I18n.t(:proposition_submitted) }
         format.json { render json: @proposition, status: :created, location: @proposition }
       else
         logger.info { "proposition is unvalid" }
         format.html { render 'index' }
         format.json { render json: @proposition.errors, status: :unprocessable_entity }
       end
     end
   end
end

rake routes

                               GET    /:locale/propositions(.:format)                         propositions#index {:locale=>/en|fr/}
                               POST   /:locale/propositions(.:format)                         propositions#create {:locale=>/en|fr/}
                               GET    /:locale/propositions/new(.:format)                     propositions#new {:locale=>/en|fr/}
              edit_proposition GET    /:locale/propositions/:id/edit(.:format)                propositions#edit {:locale=>/en|fr/}
                               GET    /:locale/propositions/:id(.:format)                     propositions#show {:locale=>/en|fr/}
                               PUT    /:locale/propositions/:id(.:format)                     propositions#update {:locale=>/en|fr/}
                               DELETE /:locale/propositions/:id(.:format)                     propositions#destroy {:locale=>/en|fr/}

routes.rb

MyApplication::Application.routes.draw do
  scope "/:locale/", :locale => /en|fr/ do
    resources :propositions, only: [:index]
    resources :films, only: [:index, :show]
    resources :articles, only: [:index, :show], as: :panorama
    ...
  end
end