What's the deal to equivalent names in other languages?

I mean, I understand that, for instance, Michel is the French equivalent for Michael, Thierry for Terry etc.

But who the hell decided these English-Dutch equivalents:

Ann = Tanneken
Bartholomew = Mewis
Sophia = Fytje

'Oh, yes! We have Sophia's over here too. Except we call them 'Fytje'

'Bartholomew? Sorry? I don't recognise that as a name. I SHALL CALL YOU MEWIS.'