I am a Spanish teacher and I am planning to move to Kraków and I would appreciate any information regarding working conditions as a language teacher, mainly in language schools and Universities.
In respect to Spanish, there's not a huge demand. It's seen as a "fun" language by people - not as a serious language. In this respect, finding one school to hire you is going to be easier said than done - though it depends on your own qualifications and experience. One thing that's certain is that you'll enter at the bottom irrespective of how much experience and knowledge you have. But really, don't expect to be offered a full time job from outside Poland.
1) How is the average wages as a teacher in a language school (in zlotys)?
For Spanish, about 25-30zl an hour, even as a native.
2) How much money (average) one needs to live in Kraków/Poland?
Depends on what standard of living you expect. But in general, assume that you need to have, as a minimum, 2000zl in your pocket a month to be able to have any chance of surviving.
4) Are teachers normally freelancers or the school hires them and takes care of social security expenses (a mirage in other countries)?
It depends. You can sometimes find schools willing to pay your social security expenses - though don't rely on it. The norm is that they don't (you can start a business to pay them - works out much more cost efficient to do it this way), though.
5) How much do teachers charge for private lessons?? (30 zl??40 zl??)
Private Spanish lessons aren't particularly plentiful, and there's plenty of competition - don't be surprised if you end up only getting 25/30zl an hour for them. I know in Poznan, with much less foreigners, the supply has pushed prices down to 25zl an hour for Spanish teachers. I can't imagine it'll be a different story in Krakow.