with ezmlm it is easy to set up sublists. Just create a list with a special switch, and subscribe it to the main list. Now subscribers to the sublist get all messages, but recipient address handling is done locally, without involvement of the main list.
message -> mail list -> recipient 1 us -> recipient 2 jp -> ... -> recipient 19999 jp -> recipient 20000 jp message -> main list -> recipient 1 us -> Japanese sublist -> recipient 2 jp -> recipient 19999 jp -> ...