Change the language by the click of a button (link in the admin bar).

lang_change_button.php

<?php
! defined( 'ABSPATH' ) AND exit;
/*
Plugin Name:  User Language Switcher
Plugin URI:   http://example.com
Description:  Change the language per user, by the click of a button
Author:       Stephen Harris
Author URI:   http://example.com
Contributors: Franz Josef Kaiser
Version:      0.2
License:      GNU GPL 2
*/



# PUBLIC API #
/**
 * A function returns with returns the user's selectd locale, if stored.
 * 
 * @since  0.1
 * 
 * @param  bool $locale
 * @return mixed string/bool $locale
 */
function get_user_locale( $locale )
{

	if ( $new_locale = get_user_meta( 
			 get_current_user_id()
			,'user_language'
			,true
		)
	);
        return $new_locale;
	
    return $locale;
}



if ( ! class_exists( 'UserLangSelect' ) )
{
	//add_action( 'init', array( 'UserLangSelect', 'init' ) );

/**
 * Allows the user to change the systems language.
 * Saves the preference as user meta data.
 * 
 * @since      0.1
 * 
 * @author     Stephen Harrs
 * @link       http://wordpress.stackexchange.com/questions/35622/change-language-by-clicking-a-button/57503
 * 
 * @package    WordPress
 * @subpackage User Language Change
 */
class UserLangSelect
{
	/**
	 * Instance
	 * 
	 * @access protected
	 * @var object
	 */
	static protected $instance;


	/**
	 * A unique name for this plug-in
	 * 
	 * @since  0.1
	 * 
	 * @access public
	 * @var    string
	 */
	static public $name ='sh_pick_lang';

	/**
	 * Hook the functions
	 * 
	 * @since  0.1
	 * 
	 * @return void
	 */
	public function __construct()
	{
		isset( $_POST[ self :: $name ] ) AND add_action( 'locale', array( $this, 'update_user' ) );

		add_filter( 'locale', 'get_user_locale',10);
		add_action( 'wp_before_admin_bar_render', array( $this, 'admin_bar') );
	}


	/**
	 * Update the user's option just in time!
	 * Only once mind...
	 * 
	 * @since  0.1
	 * 
	 * @param  $locale string
	 * @return $locale string
	 */
	public function update_user( $locale )
	{
		remove_filter( current_filter(), array( $this, 'update_user' ) );

		update_user_meta( 
			 get_current_user_id()
			,'user_language'
			,$_POST[ self :: $name ]
		);
		//Just in case get_user_locale runs too early
		return get_user_locale($locale);
	}


	/**
	 * The drop down for the admin bar
	 * 
	 * @since  0.1
	 * 
	 * @return void
	 */
	public function admin_bar()
	{
		global $wp_admin_bar;

		$wp_admin_bar->add_menu( array(
			 'id'    => 'user_lang_pick'
			,'title' => $this->lang_dropown()
		) );
	}


	/**
	 * Generates a list of available locales.
	 * Searches the wp-content/languages folder for files of the form xx_xx.mo
	 * 
	 * Based on Thomas "toscho" Scholz answer on the following WPSE question by Stephen Harris:
	 * @link http://wordpress.stackexchange.com/questions/57606/obtain-a-list-of-available-translations/57609
	 * 
	 * //FIXME including wp-admin/includes/ms.php in non-ms sites displays message prompting user to update network sites.
	 * 
	 * @since  0.1
	 * @uses get_available_language()
	 * @uses format_code_lang()
	 * @return string $html
	 */
	public function lang_dropown()
	{
		$langs   = get_available_languages();
		$langs[] = 'en_US';

		$locale  = get_locale();

		// For the labels (format_code_lang)
		require_once( ABSPATH.'wp-admin/includes/ms.php');

		// Build the option elements
		$elements = '';
		foreach ( $langs as $lang )
		{
			$elements .= sprintf (
				 '<option %s value="%s">%s</option>'
				,selected( $lang, $locale, false )
				,$lang
				,format_code_lang( $lang )
			);
		}

		// Build the form
		$html = sprintf(
			 '<form method="post"><select onchange="javascript: form.submit();" name="%s">%s</select></form>'
			,self :: $name
			,$elements
		);

		return $html;
	}
} // END Class UserLangSelect
$user_language_switcher = new UserLangSelect();
} // endif;

Ehm, ok. Forget extending the class. I read through again it and its simply not worth the effort. Too much specific stuff in there.