How should we call the official language of Iran when we speak English?
It is usually referred to as Persian, although some call it Farsi.
Similar to many other issues related to language, the choice of terminology in this case is a very sensitive one, and has been the topic of hot debates. Let's see if we can come up with some sort of middle ground based on facts.
Persian is the English version/translation of Farsi. So, it makes sense to call it Persian when we speak English. How would we then call the other two variants of the same language, namely Dari and Tajiki? If we label the three as Persian, Dari, and Tajiki, we will miss the tight genetic relationship between them: they are all derived from one single parent, and are linguistically variants of the same language. They are definitely not three different languages.
One solution is to employ Persian for Farsi, and call the other two variants Tajiki-Persian and Dari-Persian. This type of labeling clearly shows the relationship between the three variants, given the fact that all three variants are derived from Middle Persian or Farsi-ye Miyane.
But wait, the unmarked usage of Persian, referring to Farsi, gives the impression that this variant is the main version, and the other two have derived from it. This is not true linguistically though: the syntactic properties of Tajiki and Dari, for example, are closer to the syntax of the earlier stages of Persian than Farsi is.
A second solution is to follow the existing tradition used for other languages: the two major variants of Portuguese, for example, are called European Portuguese versus Brazilian Portuguese. British English versus Standard American English is another example. Given this analogy, we will arrive at Iranian Persian (for Farsi), Tajiki Persian, and Dari (Afghani) Persian.
I don't see any problem with this solution.
However, there is a simpler solution. Namely, we call the language simply Persian in our every day practice, referring to all three variants . When there is a need to specify which variant we are referring to, as it is definitely the case in linguistic analyses, we specify them as Farsi, Tajiki, and Dari, still under the cover term Persian.
In other words, Persian is used as a cover term in every day language, while Farsi is used in specialized academic/linguistic analyses.
This is how I have employed these terms over the last two decades. I am open though to any suggestion that is based on facts, rather than emotions. (Karimi@u.arizona.edu)