How do i change the menu language into Farsi (persian)?

Is it possible to make leopard into a farsi OS?

No. You can find the list of 18 possible OS languages in its tech specs (Leopard and Snow Leopard are the same in this regard):

http://www.apple.com/macosx/specs.html

To tell Apple you want this, go here:

http://www.apple.com/feedback/macosx.html

The language settings in preferences sets the sort order for the languages used. Essentially, it searches in order for the languages in whatever library/application and uses that, if available. I don't know if the OS X system software comes with a Farsi translation. I'm guessing not as Apple doesn't have a store in Iran and as far as I know isn't really sold anywhere but a couple of computer stores in Tehran. Thus, the Farsi-speaking audience for their products is very small.

That said, it's fairly straight-forward to make translations for the software. I'm sure you don't want to go to the hassle, but it doesn't mean that another person hasn't. Try googling (in Farsi) to see if you can find someone that has done the translation and has it available for download.