Do you really think ancient writers were retarded at a point to mix Germany with Iran?
There is nothing widely believed. Linguists have a high tendencies at laughing to people who makes ancestral links with some apparent and flawed linguistic relationship. There was never Germans in west or east Iran, there were no Celts in Iran neither.
Ancient writers didn't mix anything, in the ancient times a country with the name of Iran didn't exist in the west Asia, Iranians who migrated and conquered this land, gave this name to it, the same thing can be said about Turkey and Turks, and of course Germany and Germanic people.
I know it is widely believed that Iran has been always the land of Iranian people or Germany the land of Germanic people but as a historian I don't care what nationalists say, the only important thing for me is Evidence, whether historical, archaeological, linguistic, genetic, cultural, ...