Tahran is a city or town in the Azadi Empire (presumably on the Far Western continent). One of the soldiers at night in Marcuria says to April "I wonder what my betrothed is doing right now. She lives in Tahran. She told me she'll wait for me, even though I'll be here for many years."
This name was undoubtedly inspired by the real-world Tehran capital of Iran. This is just one of many Persian-influenced aspects of the Azadi, among their architecture, theocracy, and the name 'Azadi' itself.
During production of the game the name "Taheran" was used as a placeholder until Ragnar finally decided on the name "Azadi", although http//ragnartornquist.com/?p=104/ apparently it was never intended for the final cut. It is still visible on some pieces of concept art.
Although the name was probably used as a placeholder because of its Persian connection and the Azadi's obvious resemblance to the current Iranian theocracy, the popular etymology of the city's name may shed some light on the conception of the Empire. While now discredited by scholars, for some time the most widely accepted origin of Tehran (formerly Tahran) was 'one who dwells under the ground', no doubt due to Yaqut's (fl. early C.13th) description of the inhabitants of the time dwelling in shafts and caves around the town (attested as late as 1818). Given the Azadi's connection via the Prophet to the Underground Caverns beneath Marcuria, and the obvious link to the newly built Azadi Tower, there is some food for thought. It seems unlikely that the Azadi are connected to the ancient inhabitants of the ruined city, however.