i18n, URLs and the impact on SEO

I just spent the past week internationalising my site. Currently, my plan is to automatically detect what language to use via the users browser settings. For example, if the user has their language set to Spanish, the site will display in Spanish without any changes to the URL structure. I prefer this way as it's much cleaner.

However, I'm concerned that if I structure the site like this, Google will think it's only in English. If that's the case, it won't rank for other languages in non-English countries. The obvious alternative would be to use parameters (?=lang) in the URL. But if I do that, will Google view links to site.com and site.com?=es as different pages and pass link juice separately? I would much prefer there be one main page that collects all link juice, as opposed to dividing it up amongst multiple language versions.

Would canonical tags fix this issue? So if someone links to site.com?=es and the canonical tag is set to site.com, would link juice pass to site.com and have a positive effect on Spanish searches? In Mexico, would site.com?=es rank or would site.com rank?