My name is Fereshteh Davaran and I am writing my Ph.D. dissertation in the NES department at U. C. Berkeley and teaching Persian in Diablo Valley College. I wanted to ask you, as Iranians, not to use "Farsi" when you refer to Persian language in an English text.
Persian is the only language that is currently called by three different names (Farsi, Tajik and Dari) in English. You do not see anybody calling German, Almani or Deusche in English. You do not hear anybody call English, Irish or Australian or for that matter American.
Categorizing languages has a scientific method. According to Linguistics, the Persian language belongs to the Iranian branch of Indo-Iranian languages. The Iranian branch is composed of many languages such as Persian, Sughdi, Kurdish, Parthian, etc. The Persian branch has different dialects such as Tajik, Dari, Farsi, Isfahani, etc.
To call Persian, Farsi is just as bad as calling Persian Gulf, Arabian Gulf or even the Gulf. In the absence of an interested government, we Iranians have to defend our heritage more vigorously.
"Persia" is what Greek historians called Parsis at the time of the Achaemenids, and like all historic proper names its antiquity is its best defense. Over the centuries "Persian" was used to refer to the whole country of Iran and therefore could be used interchangeably with Iranian.
Fars and Farsi is the Arabic form of Parsis and Persia. Since Arabs did not have p sound, they turned Pars to Fars. Calling the Persian language by the three names of Farsi, Dari and Tajik is quite a recent phenomenon. As I said, linguists have agreed to call the language of Darius and Cyrus Old Persian, the language of Sasanids, Middle Persian and our language "Persian," which makes it the grand child of Old Persian and the Child of Middle Persian (Pahlavi).
All three stages of Persian language (old, middle and present) belong to Iranian branch of Indo-Iranian languages. Dari, Tajik, Farsi, Isfahani and Khurasani are different dialects of the Persian language, unlike Kurdish and Sughdian which are different languages in the Iranian branch of Indo-Iranian languages. Would it make sense to call Arabic, Iraqi or Egyptian, although they are three different dialects and have many differences? Would the Arabs allow it?