appending to a restful resource a custom format

Hi everyone,
I would like to add the format and a custom field to all my restful
resources, without touching my application (I don't want to change all
url_path to format_url_path to make things clear).

For being more specific I want to add :format.:lang to my restful
resources, so i can have /posts/1.html.en that map to the english page
and /posts/ that map to the italian one. the lang variable is
setted by session or cookies.

The i18n part of my app is done, everything works. I just would like to
have pretty urls, nice even for caching...

Thank you a lot!