Mirjam van Olst did a great post about changing the language of an existing SharePoint Site. She tried several different things like:
Saving the site as template didn't work, because you can only see the saved site when you create a site in the original language of the site
Using backup and restore didn't work, the site kept it's original language
Changing the language through the object model didn't work, because the Language property of the SPWeb object is read-only
She finally did a change directly in the database of SharePoint which worked great! But there is a catch. It does not work for already created lists and libraries. Secondly i would nobody recommend to do any changes directly in the database. It is also not supported by Microsoft!!
On the other hand, since this is so easy to do, would it not be great when Microsoft implements such a method into the Central Administration? I hope this blog article of Mirjam will trigger Microsoft to do so....
Thanks Mirjam for the post.
http://www.sharepointblogs.com/mirjam/archive/2008/04/29/changing-the-language-of-an-existing-sharepoint-site.aspx