Multilingual Page with Multisite: Seo Problems?

Hey,

we would like to set up a multilingual webpage. At the moment, we are not sure whether to use the WPML plugin or WP’s Multisite feature.
I would rather like to use multisites to be more inpendent of the plugin authors.

While I was doing some researach, I found this quote from someone making a comparison between using plugins and multisits:

Within this solution (multisite) you manually have to create links between the contents of each language by using cannonical tags. This is due to SEO reasons because search engines might rate your multisites as duplicate content.

Is this really true? Do I need to set cannonical tags to the main language in every header for every translated page?
If yes, is this realizable at all? I mean, I would have to set a tag in every single header, not in the main theme’s header.

What do you think about multilingual wordpress websites? Do you have some good advice?

Best regards