I know that Poland is about 1000 years old and it wasn't on the map for hundreds of years.
But the Solidarity is a political party created in the communist age to oppose the government, so why is it made into a major trait of Poland?
Solidarity gives you more government options, is this realistic? Have the ancient kings of Poland been more flexible?
Solidarnosc (Solidarity) was a Polish labor union led by Lech Walesa which protested against communism. It played a major role in the resistance against communism in Poland and it was known internationally. I would guess that is where it comes from.
Edit: About Polish kingship. Polish kings were elected by their Nobles and they were not very powerful, which is why the Polish state was weak and eventually taken over by Austria, Russia and Prussia.
This is less about their history and more about game design, but it was probably added for balance and faction diversity. The game is based in history, but features like this are usually added with game balance in mind more so than 100% historical accuracy.