Rails I18n: How do I set a fallback locale?

I am on Rails 4

and I am following how to use rails i18n fallback features

My site only has 2 lang, zh

and en

, and I want another language, for example de

, fr

to fall back to en

, so I installed as suggested above:

 config.i18n.fallbacks =[:en]

      

        
        
        
      

    

But when I visit http://localhost:3000/?locale=de

, he still reports "de" is not a valid locale

. ( locale

for en

and zh

works great)

How can I fix this?

UPDATE: this is how I get locale

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