The technique is to first buy Xbox Live gift cards, and then use them to buy games on foreign stores.
This technique is the simplest, but unfortunately some stores (Colombia, Argentina, etc) doesn't offer gift cards.
Gift cards are zoned.
If you want to buy a game on such Canadian store, you must purchase a gift card in Canadian dollars (CAD). (Do not confuse with American dollars : USD !)
The most reliable site to buy a gift card Xbox Live is Microsoft's official website.
Example for Canadian 100$ gift card.
Warning ! Some gift cards sold on the official Microsoft site are not digital, and therefore are not sent by e-mail but by postal address.
This is the case of Hong Kong gift cards (HK$), if you buy one on this site, the gift card will be sent to a Hong Kong address. So in the end, you will never received it.
Here is a page listing all websites that sell digital gift cards : Where buy Xbox Live gift cards
What you should know is that your wallet are specific to each store (country).
If you credit 100 Canadian dollars to your account. You can't buy games on a store other than Canada with the $100 CAD.
Once purchased, your game will be available directly on your console in the language of your country.