LocaleSessionFilter.php

<?php
namespace App;

use Cake\Event\Event;
use Cake\I18n\I18n;
use Cake\Routing\DispatcherFilter;
use Locale;

class LocaleSessionFilter extends DispatcherFilter
{
	protected $_locales = [];

	public function __construct($config = [])
	{
		parent::__construct($config);
		if (!empty($config['locales'])) {
			$this->_locales = $config['locales'];
		}
	}

	public function beforeDispatch(Event $event)
	{
		$request = $event->data['request'];

		$locale = Locale::acceptFromHttp($request->header('Accept-Language'));
		$language = $request->session()->read('Config.language');

		if(!$language || !in_array($language, $this->_locales)) {
			if($locale && in_array($locale, $this->_locales)) {
				$language = $locale;
			}
		}

		if($language && in_array($language, $this->_locales)) {
			$locale = $language;
		} else {
			$language = $locale = 'en_EN';
		}

		$request->session()->write('Config.language', $language);
		I18n::locale($language);
	}
}
// in your bootstrap.php add:
DispatcherFactory::add('App\LocaleSessionFilter', ['locales' => ['de_DE', 'en_EN']]);