If you're referring to the language that you're seeing when you're logged into Xbox LIVE that language is set by the country/region you selected during the creation of your account. If you wish to change the language of the Xbox LIVE account please visit this
If you're referring to the language on the console without being logged into Xbox LIVE you can follow the directions below to change that:
Press the Guide button on your controller, go to Settings, and then select System Settings.
Select Console Settings.
Select Language and Locale.
Select the language that you want to use.
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