It’s a good practice to declare the primary language of the document in the html element:

<html lang="en">

If no other lang attribute is specified in the document, it means that everything (i.e., element content and attribute text values) is in that language.

If the document contains parts in other languages, these parts should get their own lang attributes to "overwrite" the language declaration.

The lang attribute is used to specify the language of element content and attribute text values:

<p lang="en">The content of this element is in English.</p>

<p lang="en" title="The value of this attribute is also in English.">The content of this element is in English.</p>

The language declaration gets inherited:

<div lang="en">
  <p>This element contains English content.</p>
  <p title="This attribute, too.">Same with this element.</p>
</div>

You can "overwrite" a language declaration:

<p lang="en">This English sentence contains the German word <span lang="de">Hallo</span>.</p>

You can "overwrite" a parent element's language declaration by introducing any element apart from applet, base, basefont, br, frame, frameset, hr, iframe, meta, param, script (of HTML 4.0) with an own lang (opens new window) attribute:

<p lang="en" title="An English paragraph">
    <span lang="de" title="A German sentence">Hallo Welt!</span>
</p>

It is possible to add the attribute hreflang (opens new window) to the elements <a> and <area> that create hyperlinks. Such it specifies the language of the linked resource. The language defined must be a valid BCP 47 (opens new window)^[1] language tag.

<p>
    <a href="example.org" hreflang="en">example.org</a> is one of IANA's example domains.
</p>

1. ↑ IETF Network Working Group: RFC 5646 Tags for Identifying Languages (opens new window), IETF, September 2009

Syntax

Remarks

The value of the lang attribute must be a valid BCP 47 language tag or the empty string (if the language is unknown).

The BCP 47 (opens new window) language tags are listed in the IANA Language Subtag Registry (opens new window).

Accessibility

The relevant WCAG 2.0 Success Criteria are:

• 3.1.1 Language of Page (opens new window)
• 3.1.2 Language of Parts (opens new window)

The related WCAG 2.0 Techniques are:

• H57: Using language attributes on the html element (opens new window)
• H58: Using language attributes to identify changes in the human language (opens new window)