Selling the house would take to long!
Why? You can do everything in a day if funds are available.
1. IS IT POSSIBLE TO GET A HYPOTHEC FROM A POLISH BANK OF A POLISH REAL ESTATE IF YOU ARE A GERMAN CITIZEN?
Yes, although it's going to take several weeks - and you might find that the banks will do very thorough checking of you.
2. IF "YES", HOW MANY PERCENT (%) OF THE MARKET VALUE OF THE REAL ESTATE IT WOULD BE?
No idea. Banks tend to make this stuff up as they go along, especially for esoteric cases.
If you really need cash quickly, it shouldn't be a problem to sell the place for 50% of the market value quickly..