Hi Ozgur, this might look a bit confusing as it is not part of the site object. However there is a simple way. You just need to update settings value. Please see code below how to achieve this >
var siteInfo = new SiteInfoIdentifier(1);
var setting = SettingsKeyInfoProvider.GetSettingsKeyInfo("CMSDefaultCultureCode", siteInfo);
setting.KeyValue = "en-US";
setting.Update();