BusinessLogicVariable.php

<?php
namespace Craft;

class BusinessLogicVariable
{
    public function getRequestedLocaleId()
    {
        // Return the current locale ID if any path is requested
        if (craft()->request->getUrl() !== '/') {
            return craft()->locale->id;
        }

        // Get the browser's preferred languages
        if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE']) && preg_match_all('/([\w\-_]+)\s*(?:;\s*q\s*=\s*(\d*\.\d*))?/', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $matches, PREG_SET_ORDER)) {
            $languages = array();
            $weights = array();

            foreach ($matches as $match) {
                $languages[] = strtolower(substr($match[1], 0, 2));
                $weights[] = !empty($match[2]) ? floatval($match[2]) : 1;
            }

            // Sort the languages by their weight
            array_multisort($weights, SORT_NUMERIC, SORT_DESC, $languages);

            // Find the first with corresponding locale
            foreach ($languages as $language) {
                if (array_search($language, craft()->i18n->getSiteLocaleIds()) !== false) {
                    $matchingLocale = $language;
                    break;
                }
            }
        }

        // Default to German
        if (!isset($matchingLocale)) {
            $matchingLocale = 'de';
        }

        return $matchingLocale;
    }
}