Fluent::with_locale PHP Method

    public static function with_locale($locale, $callback)
    {
        // Check and set locale
        if (self::$_override_locale) {
            throw new BadMethodCallException("Fluent::with_locale cannot be nested");
        }
        if (!in_array($locale, self::locales())) {
            throw new BadMethodCallException("Invalid locale {$locale}");
        }
        self::$_override_locale = $locale;
        DataObject::flush_and_destroy_cache();
        // Callback
        $result = call_user_func($callback);
        // Reset
        self::$_override_locale = null;
        DataObject::flush_and_destroy_cache();
        return $result;
    }

 /**
  * Test versioning of localised objects
  */
 public function testPublish()
 {
     // == Setup ==
     Fluent::set_persist_locale('fr_CA');
     Versioned::reading_stage('Stage');
     // Create new record in non-default locale
     $id = Fluent::with_locale('es_ES', function () {
         $page = new Page();
         $page->Title = 'ES Title';
         $page->MenuTitle = 'ES Title';
         $page->write();
         return $page->ID;
     });
     // == Check stage ==
     // Check that the record has a title in the default locale
     $page = Versioned::get_one_by_stage("SiteTree", "Stage", "\"SiteTree\".\"ID\" = {$id}");
     $this->assertEquals('ES Title', $page->Title);
     $this->assertEquals('ES Title', $page->MenuTitle);
     // Check that the record has a title in the foreign locale
     $record = Fluent::with_locale('es_ES', function () use($id) {
         $page = Versioned::get_one_by_stage("SiteTree", "Stage", "\"SiteTree\".\"ID\" = {$id}");
         return $page->toMap();
     });
     $this->assertEquals('ES Title', $record['Title']);
     $this->assertEquals('ES Title', $record['MenuTitle']);
     // == Publish ==
     // Save title in default locale
     $page = Versioned::get_one_by_stage("SiteTree", "Stage", "\"SiteTree\".\"ID\" = {$id}");
     $page->Title = 'Default Title';
     $page->MenuTitle = 'Custom Title';
     $page->write();
     // Publish this record in the custom locale
     Fluent::with_locale('es_ES', function () use($id) {
         $page = Versioned::get_one_by_stage("SiteTree", "Stage", "\"SiteTree\".\"ID\" = {$id}");
         $page->doPublish();
     });
     // == Check live ==
     // Check the live record has the correct title in the default locale
     $page = Versioned::get_one_by_stage("SiteTree", "Live", "\"SiteTree\".\"ID\" = {$id}");
     $this->assertEquals('Default Title', $page->Title);
     $this->assertEquals('Custom Title', $page->MenuTitle);
     // Check the live record has the correct title in the custom locale
     $record = Fluent::with_locale('es_ES', function () use($id) {
         $page = Versioned::get_one_by_stage("SiteTree", "Live", "\"SiteTree\".\"ID\" = {$id}");
         return $page->toMap();
     });
     $this->assertEquals('ES Title', $record['Title']);
     $this->assertEquals('ES Title', $record['MenuTitle']);
 }