Tat (Celtic Tuatha, Germanic Theudo, Persian Tuda)

You probably know about the Tat/Tatik people: http://en.wikipedia.org/wiki/Tats  an Iranian-speaking people who live in modern Azerbaijan, Armenia, and Russia, it is said that is the name which Turkic people gave to all Iranian-speaking people, as you read about the Tadjik people in the Central Asia: http://en.wikipedia.org/wiki/Tajik_people 

 It may have the same root^[23] as the word Tat which is used by Turkic-speakers for the Iranian-speaking population of the Caucasus. In a historical context, it is synonymous with Iranian^[24]

Of course you can find this name everywhere Iranian-speaking people live, in south in India, the name of the very famous Persian Zoroastrian family of Tata could be related to this name too.

Persian poet Rumi (Molana) usually uses the word "Tat" when he wants to refer to all Iranian languages, for example he says:

گر تات ساك و گر روم ساك و گر ترك زبان بي زباني را بياموز

Gar Tat sak o gar Rum sak v gar Tork zabani bizabani ra biamuz

If you are either a Tat, Roman or Turkic speaking person, learn to speak without tongue (speak through your heart.

Ali Abdoli, author of the book "Tat Literature" in Persian says: http://iranianboy.persianblog.ir/post/332/

"تات نامي است كه به بخش گسترده يي از جامعه ايران بزرگ اطلاق شده است. تاتها نه يك قوم و نه يك طايفه و ايل اند بلكه جماعات مختلفي از ايراني زبانان هستند "

"Tat is the name which is reffered to a large part of Greater Iran's society, Tats are neither an ethnicity, nor a clan, nor a tribe but they are different societies of Iranian-speaking peoples."

For this reason I think "Tat" relates to the Persian word "Tuda" (Tudeh) which means "a large number of people", in modern Persian Tudehi means "Communist" or "Socialist".

I think this Persian word has the same Indo-European origin of Celtic Tuatha and Germanic Theudo, for example about Dutch, you can read here:

The origins of the word Dutch go back to Proto-Germanic, the ancestor of all Germanic languages, *theudo (meaning "national/popular"); akin to Old Dutch dietsc, Old High German diutsch, Old English þeodisc and Gothic þiuda all meaning "(of) the common (Germanic) people".