One URL, multiple languages, SEO-friendly?

[eluser]popovich[/eluser]
This is the point — I'd be glad to stay with the same URL but to have the language (=content) selected on acccept-lang or cookie. And to make google index the content in all three languages.
It would be a pity if I had to re-write the code and change URLs for that.

My guess is that google detects ".../in-german" link only because I have manually added and submitted the sitemap with it. The XML-machine would not want to notice those links, actually.

Any ideas how to design the redirect, so the bots pay attention to it?