I note that Google's translation service only operates when there is a significant amount of text on the page to be translated; when it's spidered, the page is read and a guess is made about which language is written in. If it's confident enough with the guess, it will offer a link to a translation.
That seems to be a fairly heavy load though.
I ran a search to bring up some German pages, and looked at the link provided for the translation service. It began:
http://translate.google.com/translate?hl=en&sl=de
and followed with the URL of the page.
I mention this because (a) it suggests the problem isn't trivial, and (b) if you can get the request past Google (it does watch out for this sort of thing, however) and want a wildly complicated kludge, you could "search" for the page and use search modifiers like site: and allinurl: to make sure the page you want to look at is the one you're after, then see if the returned result has a Translate link. If it does, parse that for the source language.
According to the HTML specification, 'The last two letters of the [doctype] declaration indicate the language of the DTD. For HTML, this is always English ("EN").' In other words, "EN" refers to the language the DTD is written in, not the language of a document written to that DTD.
Speaking of the spec, with luck the site developer may have paid attention to Chapter 8.
