Routing for bilingual website

[eluser]steelaz[/eluser]
Recently I had to to a bilingual website. To change active language I created a controller that adds a session variable for user. Constructor then reads the variable and acts accordingly. The problem I'm in right now is that this setup is not SEO friendly. I think I need my URLs to look something like index.php/en/controller/method/ for search engines to spider the whole website. How can I set this up in CI routing?