CHange windows 7 system language

How can you change the system language (ie the language for the welcome screen and the default for new accounts) for a win7 computer?

Google search answers that you need to have win7 Ultimate and download the language pack from optional items in windows update. That's done.

I know how to change display language for a user (through "Region and Language" window. What I want is to change the default system language and uninstall the old one.