There are a couple of Polish platforms, including Fellow Finance. Most significantly Mintos now also operate, as you say, in Poland. They are established, and based in the Baltic's. It's likely the area will grow, however there's some resistance to it in the market, and lobbying against regulation, by established lenders. As far as regulation is concerned, it's perhaps safer to do peer to peer lending in more established markets where it's easier to collect delinquent debts.
I use Funding Circle and Thin Cats and have done for years. It has (especially Funding Circle) been a worthwhile home for savings. They lend only to business which is a plus, however currently I would only do this in the U.K. where the market is big, established, and relatively easy to collect a lender's or their guarantors' debts.