While I'm not a big fan of Sixt, I do find some of the complaints unreasonable.

The comments on the website is for humans to read. The system does not know you are going to drive it to Poland (or for that matter drive it all the way to China if you wanted to) based on the comments section. It only goes by pickup and drop off location entered. NO car hire company has developed a website that can interpret customer intentions on the comments field. Your expectation that the website allowed such a booking against company policy is unfair.

Nothing is stopping you from driving a PTMR to Poland other than no insurance coverage should the car be lost/damaged/involved in accident.

Sixt Germany does print contracts in English. All you have to do is ask.