Well the make and model of the laptop will also be needed, since some of the manufacturers also like to tinker with Windows to their own liking, and things like changing the way it speaks to you can depend on this.
The best way to swap the language of any computer is to restore it to the way it was when it was first turned on, since that is when it was set this way in the first place. That can be pretty rough on the machine though, as this also tends to wipe out any data you had prior. The simple way is what br1ng3r said, but what happened when you tried this tends to lead me to think it may have been modified by the manufacturer, and the only option would be to either live with the way it runs now, or save everything and run the restoration program that should have come with or been made for this computer.
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