Source

File: wp-includes/general-template.php

function get_language_attributes( $doctype = 'html' ) {
	$attributes = array();

	if ( function_exists( 'is_rtl' ) && is_rtl() ) {
		$attributes[] = 'dir="rtl"';
	}

	$lang = get_bloginfo( 'language' );
	if ( $lang ) {
		if ( 'text/html' === get_option( 'html_type' ) || 'html' === $doctype ) {
			$attributes[] = 'lang="' . esc_attr( $lang ) . '"';
		}

		if ( 'text/html' !== get_option( 'html_type' ) || 'xhtml' === $doctype ) {
			$attributes[] = 'xml:lang="' . esc_attr( $lang ) . '"';
		}
	}

	$output = implode( ' ', $attributes );

	/**
	 * Filters the language attributes for display in the 'html' tag.
	 *
	 * @since 2.5.0
	 * @since 4.3.0 Added the `$doctype` parameter.
	 *
	 * @param string $output A space-separated list of language attributes.
	 * @param string $doctype The type of HTML document (xhtml|html).
	 */
	return apply_filters( 'language_attributes', $output, $doctype );
}