Fluent::default_locale PHP Method

    public static function default_locale($domain = null)
    {
        if ($domain === true) {
            $domain = strtolower($_SERVER['HTTP_HOST']);
        }
        // Check for a domain specific default locale
        if ($domain && ($domains = self::domains()) && !empty($domains[$domain])) {
            $info = $domains[$domain];
            if (!empty($info['default_locale'])) {
                return $info['default_locale'];
            }
            // With no explicitly set default_locale use the first locale assigned
            if (!empty($info['locales'])) {
                return reset($info['locales']);
            }
        }
        return self::config()->default_locale;
    }

 public function updateRelativeLink(&$base, &$action)
 {
     // Don't inject locale to subpages
     if ($this->owner->ParentID && SiteTree::config()->nested_urls) {
         return;
     }
     // For blank/temp pages such as Security controller fallback to querystring
     $locale = Fluent::current_locale();
     if (!$this->owner->exists()) {
         $base = Controller::join_links($base, '?' . Fluent::config()->query_param . '=' . urlencode($locale));
         return;
     }
     // Check if this locale is the default for its own domain
     $domain = Fluent::domain_for_locale($locale);
     if ($locale === Fluent::default_locale($domain)) {
         // For home page in the default locale, do not alter home url
         if ($base === null) {
             return;
         }
         // For all pages on a domain where there is only a single locale,
         // then the domain itself is sufficient to distinguish that domain
         // See https://github.com/tractorcow/silverstripe-fluent/issues/75
         $domainLocales = Fluent::locales($domain);
         if (count($domainLocales) === 1) {
             return;
         }
     }
     // Simply join locale root with base relative URL
     $localeURL = Fluent::alias($locale);
     $base = Controller::join_links($localeURL, $base);
 }