after creating a site you can go to: Site Actions / Site Settings / Site Administration / Language Settings and select some alternate language(s) if you've got some language pack(s) installed, but the default one is fix and you can't cange it anymore. Is there a way or some settings.file where you can change the default language?
There is no supported way of changing the language of a site.
Though I am not sure about this, you can play with powershell and try to do this. Look at
SPWeb.UICulture property and try to set it to some other language and update web.
Also look at this discussion: How to change the default site language?