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You can try to use Adam Najmanowicz tool http://blog.najmanowicz.com/2009/04/06/advanced-language-manipulation-tool-for-episerver/
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I have not tried the tool in CMS 6 yet, but the tool that Marek mentions do a good job of changing master language. I used it a few days ago on a R2 site and it worked great. Might have to do some sort of changes for CMS 6 I don't know, but try it with a backup of your database (wich you should always have) and see how it goes. Please tell us here too how it goes. =)
Is it possible to change the Default language for a page? We have changed from Swedish to English but for pages created before the change the Default language is still Swedish.