he 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>
ISO 639-1 defines abbreviations for languages. In HTML and XHTML they can be used in the lang and xml:lang attributes.