To write text in Farsi, type letters according to the character map.
For example, type airan (ایران), thran (تهران), or farsi (فارسی).

If you have a Persian keyboard then this feature is of no use to you. Just go directly to http://www.google.com/intl/fa/

This feature works with Internet Explorer, Win2K or higher. Not yet compatible with FireFox.