How to add new language in Magento?

Store Views are best used to display a Store in multiple languages.

To create a new Store View, click Create Store View in the upper right corner.

Select the Store to which this Store View will be associated. The Code and Sort order fields work like those for Websites.

For a Store View to be visible on the front-end, it must be Enabled. You can remove it from the front-end without deleting it by selecting Disabled.

You can set unique Locales for each Store View. Simply navigate to System > Configuration, and select the General tab from the left column. In the Current Configuration Scope drop-down above, select the desired Store View. Uncheck the Use website checkbox next the Locale settings, and select the Locale you prefer. This Locale will only apply to this Store View.