youdontknowme No idea about Poland, but that is not how saunas work in Finland or Germany. Swimsuits in the sauna are seen as unhygienic.
In Poland, all people more professionally dealing with this topic recommend staying in the sauna naked. Poles, however, are much more shy about nakedness than Germans or Scandinavians, they resemble Americans in this matter, which is why a lot of people - especially women - wear a swimwear in the sauna.