static function choose_site_locale($langsAvailable = array()) {
   $siteMode = Director::get_site_mode(); // either 'cms' or 'site'
   if(self::$current_locale) {
      i18n::set_locale(self::$current_locale);
// HINT: Windows and Linux compatibility. First locale native name is used like "German",
// second locale in short form like "de_DE.utf-8" is used
      setlocale(LC_ALL, i18n::$common_locales[Translatable::get_current_locale()], Translatable::get_current_locale() . ".utf8");
      return self::$current_locale;
   }

   $session = new Session($_SESSION);

   if(((isset($_GET['locale']) && !$langsAvailable) || (isset($_GET['locale']) && in_array($_GET['locale'], $langsAvailable))) && $_GET["locale"]) {
      // get from GET parameter
      $session->inst_set("locale", $_GET['locale']);
      self::set_current_locale($_GET['locale']);
      $session->inst_save();
   } else if ($session->inst_get("locale")) {
      self::set_current_locale($session->inst_get("locale"));
   } else if ($session->inst_get("loggedInAs")) {
      $member = Member::get_by_id("Member", $session->inst_get("loggedInAs"));
      if ($member) {
         $locale = $member->Locale;
      } else {
         $locale = self::default_locale();
      }
      self::set_current_locale($locale);
   } else {         
      $locale = self::default_locale();
      
      // First try to detect browser preferred language
      if(isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
         $locales = array();
         
         $list = explode(',', $_SERVER['HTTP_ACCEPT_LANGUAGE']);
         foreach($list as $part) {
            $priority = '';
            @list($_locale, $priority) = explode(';', $part);
            if ($priority) {
               $priority = substr($priority, 2);
            } else {
               $priority = 1.0;
            }               
            
            if (preg_match('/^([a-zA-Z]+)$/', $_locale, $match)) {
               if (isset(i18n::$likely_subtags[$match[1]])) {
                  $_locale = i18n::$likely_subtags[$match[1]];
               } else {
                  $_locale = self::default_locale();
               }
            } else if (preg_match('/^([a-zA-Z]+)-([a-zA-Z]+)$/', $_locale, $match)) {
               $_locale = $match[1] . '_' . strtoupper($match[2]);
            } else {
               $_locale = self::default_locale();
            }
            
            if (in_array($_locale, $locales) == false) {
               $locales[$priority] = $_locale;
            }
         }
      
         // Sort browser locales by priority
         
         krsort($locales);
         
         // Try to find best matching browser locale
         
         foreach($locales as $_locale) {
            if ($langsAvailable) {
               if (isset($langsAvailable[$_locale])) {
                  $locale = $_locale;
                  break;
               }
            } else if (self::$allowed_locales) {
               if (in_array($_locale, &self::$allowed_locales)) {
                  $locale = $_locale;
                  break;
               }
            } else {
               if (isset(i18n::$all_locales[$_locale])) {
                  $locale = $_locale;
                  break;
               }
            }
         }
      }
      self::set_current_locale($locale);
   }
   i18n::set_locale(self::$current_locale);
   setlocale(LC_ALL, i18n::$common_locales[Translatable::get_current_locale()], Translatable::get_current_locale() . ".utf8");
   return self::$current_locale;
}