SEO implications of having multiple languages available at same URL

Here's Google's own tips for multilingual sites. In summary:

• Make sure the page language is obvious by sticking to one language per page.

• Keep the content for different languages on separate URLs. Don’t use cookies to show translated versions of the page. Consider cross-linking each language version of a page.

• Tell Google if your site is targeting a particular geographic region.

• Use robots.txt to block search engines from crawling automatically translated pages on your site.