creating routes an discretionary route prefix

how i origination routes recognize an discretionary prefix parameter follows:

/*lang/controller/id

in lang biased optional, default value it's specified url:

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

edit

ideally, i'm looking opposing controllers actions mapping namespace:

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

code1