EDIT: Speaking of which, Wikipedia seems to think he may be eligible for parole in a week or so. O_o
I understand 'Hans' is related to 'Johannes' and is sometimes anglicized to 'John' (I believe one of my great grandfathers did this.) One of the nice things about the name 'John' is you can change it in various ways without anybody thinking it unusual; e.g. a 'John' can also call themselves 'Jack' and nobody thinks that unusual.
On the other hand, C.S. Lewis apparently went by Jack, which isn’t a common nickname for Clive or Staples. But people generally don’t argue over nicknames. And they may not even realize a nickname actually is a nickname; it’s not common for people to ask for your legal ID, after all.
Apparently Jack is a generic name, too, for any small boy. So inns would employ a boot-Jack to help guests get their boots off, later supplanted by a gadget.
"What's your name, son?"
"Hitler."
"Howdy, Hitler, you ready to help us fight Hitler?"