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>