Recently, I had a problem with an existing SharePoint site. The problem was that the site was created in English, while it should have been created in Dutch. For changing the language with existing SharePoint site, you need to fire following queries in SharePoint database as per your requirements (i.e. WebApplication, SiteCollection).
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]]