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.
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]]