Specify Language Attribute on the root html node
Summary
From the HTML5 spec:
Authors are encouraged to specify a lang attribute on the root html element, giving the document's language. This aids speech synthesis tools to determine what pronunciations to use, translation tools to determine what rules to use, and so forth.
Read more about the lang attribute in the spec.
Head to Sitepoint for a list of language codes.
Helpful Links
<html lang="en-us">
<!-- ... -->
</html>