Enable Language Filter Plugin
You have to publish the Language Filter plugin.
You can access the plugin from Extensions->Plugins and then search por the Language Filter. You will have the following screen:
You have several options. It is important that if you want to have translated content switching when switching languages you have to set the value 'YES' for the option "Associations". If you forget to do it at first then you will have to create associations manually re-editing the items.