Account language

That's not changing your region or language. That's your locale on your console. The only way to change the account region language would be by going to the link I gave you above and following the directions under Change your Xbox LIVE account language.
You followed the directions for Change your Xbox 360 console language.

The console language does not affect the language of the game it's only for family settings and WiFi settings if you use those two things. The Xbox LIVE account language is what's going to affect the language of the game and your account.

But as I said, ultimately it's up to the developer to make it so there are multiple language options from within the games menu. Without those two options available to you, you will not be able to change the language of a.) your account (if there's no preferred
language drop down) or b.) change the language of the game (if there's no settings to change the game language).

with blackops the DLC language must match the DISC language

also the language of the DLC is limited to the region of your account offical languages

and your physical region and account region must match to download the DLC

That would be why

Your account region sets the language of any dlc or games on demand game based on your nationally supported language

Consolw language doesnt affect dlc or availability of languages in live

The region of your account and the nationally supported language dictates what languages are available in your region

Each County/Region has specific languages that are available in Xbox Live. If you're changing your Xbox Live Region to one that has multiple languages available, you'll be given the option to choose your "preferred language". If that option is not available,
that means that only one language is available for that Region.

Here’s how to change your Xbox Live account language:

Go to Billing Account Information at
live.xbox.com/.../billingaccountinformation.

Sign in using your Microsoft account.

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, in the United States).

Click Save.

if I remember correctly if you are physically in Japan you are automatically routed to Japan it's to do with your DNs not xbox