aakk9999 - 3:39 am on Nov 5, 2012 (gmt 0)
You STILL have to go by the language on the page rather than the declared language...
It is not always so simple. I had few cases where the page was completely in English (writen by a native English speaker) AND also used lang="en", but Google decided the page was in the local language of the country where the domain was hosted. There was no one word of the local language on the page.
Google takes other signals and often ignores the language on the page itself and it does get it wrong. I think this is less of the problem for domains hosted in English-speaking countries and owned by locals, but it can be a big problem for cases where non-english speaking country has a domain run in English but hosted and owned locally.
In cases like this the entry in English SERPs (google.com, or google.co.uk) gets "Translate this page", which, if clicked, makes Google translating the page from English to English. The CTR also suffers when the "Translate this page" shows unecessary.