Hello,
Indeed, after all it seems there is no way to get this behaviour with Polylang.
The way that wordpress and polylang currently working not allowing multiple slug.
I tried the php code suggest by AndyDeGroo in this post:
http://wordpress.org/support/topic/plugin-polylang-identical-page-names-in-different-languages?replies=8
but it seems it is not working anymore.
Finally there is no solution available unless switch to an other plugin such as WPML or create a Wordpress multisite.