grandmother was uneducated and can't even write her name,how can she signed?
Quite common in Poland, especially among uneducated people. Normally random relatives turn up after the death when they realise that the property was actually worth something.
We don't know as aunty never and will not show the documents
How much contact have you had with the grandmother over the years?
is there somewhere in Poland where lands and properties are registered legally and can we find out as a third person?
It is possible, but the best advice is to get a good lawyer from the beginning.
However, you should think about this in moral terms. If your partner barely saw his grandmother and took little interest in her, then he really has no moral right to anything.