How we can add persian language to our plone ?
The Persian translations only cover 18% of all Plone related texts...so you won't have much fun with Plone in Persian unless you translate the missing parts yourself.
Fortunately for you there are some translations already done, although as Andreas points out not so much.
You can read about how to translate plone user interface here:
Let us know if you need any help.
A lot of the translations are for the admin-interfase, you might not need to translate things that are 'not often seen' (as all the setup screens' if your users know english.
If you are good in english (or another 'well translated language (spanish, german maybe) you should be able to do most translations in a (very long) day. (I have done basically all the norwegian translations and I spent 'two half days' and a little bit more…
Basically, you use a (free) program, like po-editor to open the 'po'-files in plone.app.locales and translate whatever you can.
After that it is really useful if you know another language since there are some things that are really difficult to understand (what they are used for). If you know another language you can open 'the po files from those languages' and it is easier to understand