In international transaction of 30 days credit, there is a sudden currency fluctuation during the credit period, who has to bear the loss?

Payment on the due date of payment  has to be made in the designated currency. Any loss or profit after conversion of this currency into the required currency by the seller will have to be borne or accepted by the seller.

In international trade hard currency to be used and in few cases that there is credit such as if this company is an authorized dealer to that manufacturer

If it's not in the contract, I would say where ever is going to swing advantage wise.