What is the 'lang' attribute of the <html> tag used for?
I am quoting this from W3C:
Declaring language in HTML
Always use a language attribute on the
htmltag to declare the default language of the text in the page. When the page contains content in another language, add a language attribute to an element surrounding that content.Use the
langattribute for pages served as HTML, and thexml:langattribute for pages served as XML. For XHTML 1.x and HTML5 polyglot documents, use both together.Use language tags from the IANA Language Subtag Registry.
Also a good read is Why use the language attribute?.
You asked "how is this useful".
"The
<lang=>attribute can be used to declare the language of a Web page or a portion of a Web page. This is meant to assist search engine spiders, page formatting and screen reader technology"
Source: http://symbolcodes.tlt.psu.edu/web/tips/langtag.html (Wayback Machine link)
No mention of translation - but often a search engine spider will not want to parse through a document "in the wrong language" - its index file will grow (lots of new words), and the results will not be useful to the user (who cannot read the language, and who is using the wrong search terms).
The advent of smart translation technology (like Google's, referred to above) means that some search engines can see a page in one language, translate it, and figure out that someone searching for "cow" may be interested in this page that mentions "vache" and has <lang="fr">.