#users_controllers.rb
  def edit
    @user = User.find(params[:id])
    @languages = Language.find(:all)
  end

  def update
    @user = User.find(params[:id])

    respond_to do |format|
      if @user.update_attributes(params[:user])
        flash[:notice] = 'User was successfully updated.'
        format.html { redirect_to(@user) }
        format.xml  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @user.errors, :status => :unprocessable_entity }
      end
    end
   end


#edit.html.erb vista de edicion de USER
<h1>Editing user</h1>

<%= error_messages_for :user %>

<% form_for(@user) do |f| %>
  <p>
    <b>Name</b><br />
    <%= f.text_field :name %>
  </p>

  <p>
    <b>Login</b><br />
    <%= f.text_field :login %>
  </p>
    <p>
<%= collection_multiple_select('user', 'language_ids', @languages, :id, :english_name) %>
  <p>
    <%= f.submit "Update" %>
  </p>
<% end %>

<%= link_to 'Show', @user %> |
<%= link_to 'Back', users_path %>

#application_helper.rb
# no necesitas helpers adicionales en application_helper.rb

### MODELOS ####
#user.rb
class User < ActiveRecord::Base
     has_and_belongs_to_many :languages
end

#language.rb
class Language < ActiveRecord::Base
     has_and_belongs_to_many :users
end