Where is it used? The name Theodor is mainly used in German.
It is an abbreviated form of Theodora, used in English, Greek and in Ancient Greek.
How do they say it elsewhere? Theodore (In English)
If it's too long people might use In English: Theo
For the opposite sex use: In Scandinavian, Romanian, Czech, Polish, Serbian and in Croatian: Teodor (M) In Italian, Spanish and in Portuguese: Teodoro (M)