To create a multilingual ecommerce website we need to add language packages. So I am explaining how to add language pack in magento. We can install the language packs by following these steps -

1) Go to magentocommerce website and search the desired language pack.
2) Get the key to install language pack.
3) Go to system -> magento connect -> magento connect manager.
4) Login into connect manager.
5) Insert the key in Install New Extensions section.
6) Go to System -> Manage Store and create store view for installed language.
7) Now, you can see the language switcher on frontend.
8) To make the products and categories available in the language , you need to fill all details of product and category in that language.
9) While creating categories and products select store from Choose Store view dropdown.
10) Fill details in that language.
12) Now on changing the language in frontend you will see the content in selected language.