The lang global attribute helps define the language of an element: the language that non-editable elements are written in, or the language that the editable elements should be written in by the user. The attribute contains a single "language tag" in the format defined in RFC 5646: Tags for Identifying Languages (also known as BCP 47).

If the attribute value is the empty string (lang=""), the language is set to unknown; if the language tag is not valid according to BCP47, it is set to invalid.

Even if the lang attribute is set, it may not be taken into account, as the xml:lang attribute has priority.

For the CSS pseudo-class :lang, two invalid language names are different if their names are different. So while :lang(es) matches both lang="es-ES" and lang="es-419", :lang(xyzzy) would not match lang="xyzzy-Zorp!".

The full BCP47 syntax is in-depth enough to mark extremely specific language dialects, but most usage is much simpler.

A language tag is made of hyphen-separated language subtags, where each subtag indicates a certain property of the language. The 3 most common subtags are:

Language subtag

Required. A 2-or-3-character code that defines the basic language, typically written in all lowercase. For example, the language code for English is en, and the code for Badeshi is bdz.

Script subtag

Optional. This subtag defines the writing system used for the language, and is always 4 characters long, with the first letter capitalized. For example, French-in-Braille is fr-Brai and ja-Kana is Japanese written with the Katakana alphabet. If the language is written in a highly typical way, like English in the Latin alphabet, there is no need to use this subtag.

Region subtag

Optional. This subtag defines a dialect of the base language from a particular location, and is either 2 letters in ALLCAPS matching a country code, or 3 numbers matching a non-country area. For example, es-ES is for Spanish as spoken in Spain, and es-013 is Spanish as spoken in Central America. "International Spanish" would just be es.

The script subtag precedes the region subtag if both are present — ru-Cyrl-BY is Russian, written in the Cyrillic alphabet, as spoken in Belarus.

To find the correct subtag codes for a language, try the Language Subtag Lookup.

WCAG Success Criterion 3.1.1 requires that a page language is specified in a way which may be 'programmatically determined' (i.e. via the lang attribute).

WCAG Success Criterion 3.1.2 requires that pages with parts in different languages have the languages of those parts specified too. Again, the lang attribute is the correct mechanism for this.

The purpose of these requirements is primarily to allow assistive technologies such as screen readers to invoke the correct pronunciation.

For example, the language menu on this site (MDN) includes a lang attribute for each entry:

<div class="dropdown-container language-menu">
  <button id="header-language-menu" type="button" class="dropdown-menu-label" aria-haspopup="true" aria-owns="language-menu" aria-label="Current language is English. Choose your preferred language.">English
    <span class="dropdown-arrow-down" aria-hidden="true">▼</span>
  </button>
  <ul id="language-menu" class="dropdown-menu-items right show" aria-expanded="true" role="menu">
    <li lang="ca" role="menuitem">
      <a href="/ca/docs/Web/HTML/Global_attributes/lang" title="Catalan">
        <bdi>Català</bdi>
      </a>
    </li>
    <li lang="de" role="menuitem">
      <a href="/de/docs/Web/HTML/Globale_Attribute/lang" title="German">
        <bdi>Deutsch</bdi>
      </a>
    </li>
    <li lang="es" role="menuitem">
      <a href="/es/docs/Web/HTML/Atributos_Globales/lang" title="Spanish">
        <bdi>Español</bdi>
      </a>
    </li>
    <li lang="fr" role="menuitem">
      <a href="/fr/docs/Web/HTML/Attributs_universels/lang" title="French">
        <bdi>Français</bdi>
      </a>
    </li>
    <li lang="ja" role="menuitem">
      <a href="/ja/docs/Web/HTML/Global_attributes/lang" title="Japanese">
        <bdi>日本語</bdi>
      </a>
    </li>
    <li lang="ko" role="menuitem">
      <a href="/ko/docs/Web/HTML/Global_attributes/lang" title="Korean">
        <bdi>한국어</bdi>
      </a>
    </li>
    <li lang="pt-BR" role="menuitem">
      <a href="/pt-BR/docs/Web/HTML/Global_attributes/lang" title="Portuguese (Brazilian)">
        <bdi>Português (do&nbsp;Brasil)</bdi>
      </a>
    </li>
    <li lang="ru" role="menuitem">
      <a href="/ru/docs/Web/HTML/Global_attributes/lang" title="Russian">
        <bdi>Русский</bdi>
      </a>
    </li>
    <li lang="uk" role="menuitem">
      <a href="/uk/docs/Web/HTML/%D0%97%D0%B0%D0%B3%D0%B0%D0%BB%D1%8C%D0%BD%D1%96_%D0%B0%D1%82%D1%80%D0%B8%D0%B1%D1%83%D1%82%D0%B8/lang" title="Ukrainian">
        <bdi>Українська</bdi>
      </a>
    </li>
    <li lang="zh-CN" role="menuitem">
      <a href="/zh-CN/docs/Web/HTML/Global_attributes/lang" title="Chinese (Simplified)">
        <bdi>中文 (简体)</bdi>
      </a>
    </li>
    <li>
      <a href="/en-US/docs/Web/HTML/Global_attributes/lang$locales" rel="nofollow" id="translations-add">Add a translation</a>
    </li>
  </ul>
</div>

• All global attributes.
• Content-Language HTTP Header
• HTML translate attribute