How to change Live Rewards Email language?

When you migrate your Xbox account it doesn't automatically update the Rewards program. You would have to opt out of the Rewards program and re-register after a few days in order to update your information.

Here are instructions on how to change it -
support.xbox.com/.../manage-live-id.aspx

It will update itself on the Rewards program, however it won't be instant.

Will I still receive Xbox LIVE Rewards Points if I change my Windows Live ID (email)?

A. Yes. If you migrate your Xbox LIVE Rewards account to a new Windows Live ID (email), there may be up to a 96 hour delay before you can log into Xbox LIVE Rewards with your new email. You may even be prompted to re-register – just ignore this request,
but it will not hurt if you re-register. If you attempt to log in with your new email but Xbox LIVE Rewards does not recognize you, you can always check on your points using your old email. Rest assured, once the migration is complete, your points will be
moved, and you will be able to access the information with your newly linked email.

Hi Sundog512. Your Xbox Live Rewards email is taken from your Billing email information. Have you changed it at commerce.microsoft.com?

Hello,

Changing the language and locale on the console itself will only effect offline language. When you connect to Xbox LIVE it uses the countries language on the account. If your account is registered in Mexico, then you will see the dashboard in Spanish.

If it's possible to change, the country you have selected would need to have multiple language options. You can learn how to change this setting (if available for your country) by going
here and looking under "Change your Xbox LIVE account language".

Hello

If text in the Xbox Dashboard appears in another language, you may need to change your console or Xbox LIVE account language. Here’s how:

Change your Xbox 360 console language

To change the language setting on your Xbox 360 console, follow these steps:

1. Press the Guide button on your controller, go to Settings, and then select System Settings.
   
2. Select Console Settings.
3. Select Language and Locale.
4. Select Language.
5. Select the language that you want to use.

Change your Xbox LIVE account language

If your Xbox LIVE account is set to a country or region that supports multiple languages (for example, Canada), you can change your Xbox LIVE account language. Here’s how:

1. 1. Go to Billing Account Information.
      
   2. Sign in using your Microsoft account.
   3. If multiple languages are supported for your country or region, select a language from Preferred Language.
      
      Note The Preferred Language option does not appear if only one language is supported (for example, the United States).
      

1. Click Save.

It may take up to 24 hours for this change to take effect on your Xbox 360 console.

The country or region of an Xbox LIVE account cannot be changed once the account is created, but you can select a different language if more than one is available.