The HTML 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 engines and browsers.
According to the W3C recommendation you should declare the primary language for each Web page with the lang attribute inside the <html> tag, like this:
<html lang=”en”>
…
</html>
In XHTML, the language is declared inside the <html> tag as follows:
<html xmlns=”http://www.w3.org/1999/xhtml” lang=”en” xml:lang=”en”>
…
</html>
Related topic: How to design a multilingual website