My mom was German, and she ALWAYS used 'whom' when appropriate. And I think that that was because of the marked accusative and dative cases in German:

 

SUBJECT : Who "Wer"

DIRECT OBJECT (ACC): Whom "Wen"

INDIRECT OBJ (DAT):  Whom "Wem"

 

For non-masculine nouns, however, there are no changes in accusative case, but there are changes in dative case. I'm guessing that may be where that explanation came from.

 

Here is the declension (if I remember it correctly! And if not, someone will correct me!)

 

Subject: Der Mann; Die Frau; Das Maedchen

ACC (DO): Den Mann; Die Frau; Das Maedchen

DAT (IO): Dem Mann: Der Frau; Dem Maedchen

 

Linda