Creating routes with an optional path prefix

How can I go about making my routes recognise an optional prefix parameter as follows:

/*lang/controller/id

In that the lang part is optional, and has a default value if it's not specified in the URL:

/en/posts/1   => lang = en
/fr/posts/1   => lang = fr
/posts/1      => lang = en

EDIT

Ideally, I'm looking to do this across many controllers and actions by mapping a namespace:

map.namespace "*lang" do |lang|
  lang.resources :posts
  lang.resources :stories
end

ruby-on-rails ruby routes

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