It is necessary to open a bank account in Poland only if a company wants to receive a VAT refund in Poland. If a company does not intend to ask for a VAT refund the company does not need to have a bank account in Poland. The currency in Poland is PLN, therefore all VAT returns made by the polish tax administration are made in PLN.
The bank account should be in PLN to avoid expensive bank charges for currency conversion. The exception to the above rule is a refund in the VAT-REF application. In that case, the amount of VAT refund can be requested on the bank account outside of Poland. However the cost of currency conversion cannot be avoided because the refund amount is always expressed in PLN.