Mirjam van Olst of Macaw (http://www.macaw.nl) explains how to update the language of existing SharePoint sites by running a sql statement on the content datbase.

http://www.sharepointblogs.com/mirjam/archive/2008/04/29/changing-the-language-of-an-existing-sharepoint-site.aspx

For changing the language of all sites in the content database to Dutch the query would be:

UPDATE dbo.Webs SET Language = 1043

Changing the language of one site collection can be done with:

UPDATE dbo.Webs SET Language = 1043 WHERE SiteId = [[SiteCollectionId]]

And for changing the language of a single web or subsite you can use:

UPDATE dbo.Webs SET Language = 1043 WHERE Id = [[WebId]]