The following discussion has been summarised from the 20th installment of +Dumb SEO Questions Google+ Hangout.
Asked by Sam Naim
When you have a website with two languages (english and french), there are duplicate pages: one for english and one for french. Does that hurt your SEO? Would search engines penalize you for having similar pages in this case? Do you recommend using rel=”canonical”?
Will Google see my Multilingual Content as Duplicate Content?
Responses from SEO Questions Hangout
Rob Maas: There is no duplicate content when you have two languages on your site. The content is aimed at two different target audiences, so you don’t have to fear duplicate content.You can use <link rel=”alternate” hreflang=”x” etc> though it’s not necessary, Google is smart enough to figure it out themselves.
Having the same content in multiple languages will not count as duplicate content. You can use the <link rel=”alternate” hreflang=”x” etc> link, though Google will often determine the correct language and audience itself. For more information on using this link, see here: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=189077