Can the refund be paid to a foreign bank account?

Yes. You do not need to keep a German bank account open for the refund; the Deutsche Rentenversicherung transfers to accounts worldwide. What the transfer needs is complete account data: account holder, bank name and address, IBAN where the country uses it, and otherwise the account number with the SWIFT/BIC code. Two practical points are worth knowing. A payout to a non-German account can add up to two months on top of the normal processing time, and the amount arrives in euros, so your receiving bank sets the exchange rate and any conversion fee. Naming the account correctly in the application is the easiest way to avoid a second round of correspondence.