Poland is DCC country on every POS but almost all cashiers know how to opt you out once you emphasize to them using the key word "Polish" when they swipe your card.

Their English generally is poor or non-existent, but they all seem to understand when customer said "Polish" when handing out the card, the customer wanted the local currency.

Sometimes the device to swipe and the device to choose currency is 2 different devices. Witnessed something like this happened at a train ticket window. Saw the lady punched on a different device than the one she swiped my card. Good though that lady chose PLN for me. Not so at another train station where we used on 2 consecutive days and where the woman was particularly rude. She was super annoyed the day before when we gave a 100 PLN for the train tickets. The next afternoon I decided to use card, thinking it worked at train ticket window before - this time this rude woman DCCed me. Then she shrugged on my question. I truly believe she did it on purpose. For $0.20 difference, it has to be LOL moment.

The other time was at a McD inside a train station. The cashier did not know what to do with CC payment. A supervisor was called. She processed the payment very swiftly. I also noticed she swiped the card on one device and punched on another device. Unfortunately this time she chose the DCC button. I was never asked the whole time. But I think it is an innocent mistake in McD's case.

At a big Carrefour I did not know what to do after I used the correct red button on the stand alone little device (the same one that I saw in other places where the cashier did for me) to opt out, as the screen displayed another message that was quite confusing. Instead of going further I showed it to the cashier and told her "Polish". She immediately understood and punched the red button again TWICE. Receipt was in PLN.

At restaurants , the handheld device by the table handled both payment process and options on conversion. No need for separate devices. All wait staff understood what "Polish" meant - far far more effective than saying "please bill in Zloty" as I tried to do during our first couple days.
We ate many times at North Fish, a Poland only fast food chain with very good seafoods, in the food courts of shopping malls in different Poland cities. Again, the word "Polish" works.

The DCC mark up in Poland is 3%, versus the 5% experienced in Dubai earlier this year in January.

Buying train/bus tickets on the machines through out Poland never encounter DCC option and never being DCCed. No PIN was ever needed either, despite it shows how to insert the chip card. In fact the purchase experiences from ticket machines in Poland by far being the most smooth among all our travels in Europe in all the years. I am truly impressed. This is very good because the fares always seem to have cents involved. Using CC to purchase makes it so much easier when it comes to either finding the changes or getting back a bunch of changes and to keep track of them.

At Vilnius, Lithuania we ate at a chain Asian fusion restaurant called Manami. They seem to only locate in shopping centers. We ate at 2 different locations, with one super convenient being just directly across from the Radisson Blu we stayed at. The bill was always in Euro even I forgot to tell the waitress one time. Excellent foods and dirt cheap by the way.
We had lunch at another restaurant near the Town Hall and the bill also came in Euro.
Entrance fee at Trakai Castle was also in Euro. No DCC.

Did not use card in Tallinn where we only bought a day pass on the public transportation with cash as the only spend.

At Helsinki the only spend was to pay the Finnair bus to the airport. The driver processed the payment with CSR, in Chip + Signature fashion. No DCC.

Will return to South Africa in August. IIRC there was no DCC on our last 2 trips (2015 and 2016). Hopefully this time would be the same.