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The number of languages supported by the standard CJuiDatePicker implementation is limited and the language code to use is different from the application's language code.
This Wiki shows how to improve on that.
You have a multilingual application, and you want the URL of a page to be different for different languages, to account for SEO. The URL for the contact page for example should look like http://something.com/en/contact in english, and http://something.com/de/contact in german. This tutorial describes how to make it happen.
Note that currently selected language is always a part of the URL, and thus available to the application through $_GET. You don't have to use sessions variables to keep track of the language if you don't want to.
In case of a multilingual application, one might consider it a reasonable approach to store the preferred language of the user in a session variable, and after that, every time a page is requested, to check this session variable and render the page in the indicated language.
This tutorial shows a Yii-way of doing this.
We implement an event handler for the onBeginRequest event; as the name of the event suggests, this event handler will be called at the beginning of each request, so its a good place to check whether a language is provided (via post, session or cookie) and set the application language accordingly.
We also implement a simple Language-Selector Widget, which can render the language options as ajax-links or as a drop-down list.