Change language declarations in WordPress.

class.Toscho_Language_Filter.php

<?php
/**
 * Adjust the public visible language declarations.
 *
 * @author "Thomas Scholz" http://toscho.de
 * @version 1.0
 */
class Toscho_Language_Filter
{
	/**
	 * @var string
	 */
	public static $lang = 'de';

	/**
	 * Pseudo constructor.
	 *
	 * @param  string $lang The language according to …
	 * @link   http://www.evertype.com/standards/iso639/iana-lang-assignments.html
	 * @return void
	 */
	public static function setup($lang = 'de-DE-1901')
	{
		self::$lang = $lang;

		add_filter(
			'language_attributes'
		,	array ( __CLASS__, 'html' )
		);
		add_filter(
			'pre_option_rss_language'
		,	array ( __CLASS__, 'get_lang' )
		);
	}

	/**
	 * Returns the language string.
	 * @return string
	 */
	public static function get_lang()
	{
		return self::$lang;
	}

	/**
	 * Output for language_attributes().
	 * We drop the 'dir' attribute.
	 *
	 * @return string
	 */
	public static function html()
	{
		return 'lang="' . self::$lang . '"';
	}

}
// Adjust this to your needs.
Toscho_Language_Filter::setup('de-DE-1901');