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?
Though I am not sure about this, you can play with powershell and try to do this. Look at
Also look at this discussion: How to change the default site language?