I'm trying to get this working for almost a week. Tried Polylang, WPML (which actually allows having same permalinks for posts and pages, but not taxonomies, categories etc), tried hooking into template loading and template redirects, changing permalink structures etc. No results. I even explained it in details in front of Andrew Nacin and other core developers in Core blog. It is just a way WordPress works right now. It heavily relies on slugs in permalinks, that's why they must be unique.
So, the answer right now is "it's impossible". But I'm still looking for a solution, workaround or a hack to overcome these limitations. The solution must be stable and reliable as I'm building large platform. When I'll find this solution, I'll post it here.