It seems to be 8 in 'Misses' and 11 in 'Junior' size .
usatourist.com/english/tips/Womens-Sizes.html
Also do young women wear low cut jeans in Poland?
It depends how low cut ;). If you mean 'biodrówki' like this - yes.
Is straight leg popular or the new skinny leg jeans?
Both are ok, straight leg is more universal, not all women like skinny leg jeans.