以下、自らのメールの一部抜粋。
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the following pages includes how Google treats "canonical" and "alternate" tags.
International - Webmaster Tools Help":
http://support.google.com/webmasters/bin/topic.py?topic=2370587
About rel="canonical" - Webmaster Tools Help"
http://support.google.com/webmasters/bin/answer.py?answer=139394
I picked up some points which related with our site.
(a) "canonical" is not for international pages?
Google says there is no need to hide by robots.txt. And in that context, it sounds like that there might be also no need to hide by "rel=canonical".
I quated the following from "Duplicate content and international sites"(http://support.google.com/webmasters/bin/answer.py?hl=en&answer=182192&topic=2370587&ctx=topic#3).
There is generally no need to 'hide' the duplicates by disallowing crawling in a robots.txt file or by using a 'noindex' robots meta tag. However, if you're providing the same content to the same users on different URLs (for instance, if both example.de/ and example.com/de/ show German language content for users in Germany), you should pick a preferred version and redirect (or use the rel=canonical link element) appropriately.
And the page you introduced us ( http://www.seomoz.org/blog/duplicate-content-block-redirect-or-canonical ) suggests the same.
[ EDIT ] :
I wrote "canonical tag may not be for international pages", BUT IT DEPENDS.
Just before I sent this email, I realized that Google it self use "canonical" for international pages.
See the HTML source of "http://support.google.com/webmasters/bin/topic.py?topic=2370587". There is "canonical" tag with many "alternate" tags.
(b) Switching the same pages of other languages/regions.
The pull-down language option on top right of our pages would be better to link to the same page of another language.
http://support.google.com/webmasters/bin/answer.py?hl=en&answer=182192&ctx=topic#1
Consider cross-linking each language version of a page. That way, a French user who lands on the German version of your page can get to the right language version with a single click.
(c) We can use "alternate" for sitemap.xml
c.f. http://support.google.com/webmasters/bin/answer.py?answer=2620865
(d) Google Webmaster Tools "geotargeting tool":
There is another way to handle international pages.
http://support.google.com/webmasters/bin/answer.py?answer=62399
But in order to use this function, I GUESS, we should devide the registration to Webmaster Tools by each language-region.
(e) FYI: Samples of use of "rel=alternate hreflang="
http://support.google.com/webmasters/bin/answer.py?hl=en&answer=189077
<link rel="alternate" hreflang="en" href="http://www.example.com/page.html" />
<link rel="alternate" hreflang="en-gb" href="http://en-gb.example.com/page.html" />
<link rel="alternate" hreflang="en-us" href="http://en-us.example.com/page.html" />
<link rel="alternate" hreflang="de" href="http://de.example.com/seite.html" />
