What language do they speak in Iran?

The official language of Iran is Iranian Persian, also often called Farsi. Persian is the dominant language in the country, with Kurdish, Azerbaijani and other Turkic dialects making up significant minorities.

People in Afghanistan speak Persian, known there as Dari. People in Tajikistan speak Persian as well, buyt they call it Tajik. These name changes are for political reasons, related to the differences between Iranian interpretations of Islam and those of surrounding areas. The people living in what is now Iran have spoken a version of Persian since at least 600 B.C., and the version called New Persian, still in use as of 2015, has been around for at least 1,000 years.