In Poland it has been used since XIIIth century and its archaic Polish forms were: Tader, Chodor, Todor, Cader /Czader, Ceder/Czeder. The Russian form (Fyodor) is spelled Fiodor in Poland.
― Anonymous User 6/25/2017
2
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https://en.m.wiktionary.org/wiki/Teodor#Estonian
https://www.stat.ee/nimed/Teodor
Also Hungarian:
https://www.behindthename.com/name/teodor/top/hungary
https://hu.m.wikipedia.org/wiki/Tivadar_(keresztn%C3%A9v) - listed as a variant