Well,
Tania is a pet form of
Tatiana as you said, and technically, you're free to do as you like. If you're asking whether there's historical precedent to being
Tania and calling yourself
Tatiana, I'm doubting there's any significant number of people calling themselves that way.
However, there's nothing to stop you, so if it makes you happy, have at. (I know a female
Ashley called
Bob, after all, and a
Madeline sometimes called
Reid. You could start going by Azerbaijan if you wanted; the Name Police won't stop you, hee.)
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