Does anyone know if this occurs in any other language but English. And does anybody know, offhand, how it came to be so?

I can be corrected here but I think that is is primarily a function of US English. We don't really use the boys names that have been mentioned in previous posts in everyday Br English - John, Peter etc. Terms like "Dear John" letter were originally a US term (I think). Perhaps it derives from the use of terms like "John Doe" - another term that we don't use.

Maybe RhubarbC or Maverick disagrees?

Jo