[gizmo]

A polite letter informing them they are in violation of labor laws should be sufficient. Their gamble is you’re afraid to assert your rights, and if you can make clear that you understand they are guaranteed to lose a court case if it gets to that point they’ll pay you swiftly. European companies really don’t want to get a reputation of getting sued by their employees and losing, and they also don’t want to anger local judges.

Make sure your letter(s) are well informed, factual, nonemotional, and don’t make any threats (explicit or implied).