Our company is about to re-launch its range of localized sites.

Our main eCommerce site is targeting the US market, and parked under www.example.com, with its content in English.

We are launching 2 versions of our website in order to serve the Canadian and Mexican markets, which will be hosted under www.example.ca and www.example.mx.

We will also translate our content in French Canadian and Spanish, in order to allow users to view our site in the language they are most comfortable with.

I'm debating what is the best way to setup our URL to accomodate our multi-market and multi-lingual approach?

I was thinking each site would have a default language:
.com -> english
.ca -> french
.mx -> spanish

but and each site would allow the user to change the language, and this would be accomplished by a URL querystring parameter.

For instance:

* www.example.com/url-of-my-page.html
-> Page in english on the US site
* www.example.com/url-of-my-page.html?lang=fr
-> Page in french on the US site
* www.example.com/url-of-my-page.html?lang=es
-> Page in spanish on the US site

* www.example.ca/url-of-my-page.html
-> page in french on the canadian site
* www.example.ca/url-of-my-page.html?lang=en
-> page in english on the canadian site
* www.example.ca/url-of-my-page.html?lang=es
-> page in spanish on the canadian site

Do you think this is a good approach?

I'm especially concerned about duplicate content issues.
With this approach, 3 different sites (www.example.com/url-of-my-page.html, www.example.ca/url-of-my-page.html?lang=en and www.example.mx/url-of-my-page.html?lang=en) will actually show the same content.
Should we prevent Search Engines from indexing the localized version of each site, and only let them crawl each site in their default language (English for .com, French for .ca and Spanish for .mx ?).

Thank you in advance for sharing your insights on this subject, or pointing me to relevant resources.

Lothaire