Hi there,
and welcome to the forum!
1/ I know that I have to convert my EASA PPL to FAA PPL, is it complicated? How much it cost?
No, actually it is quite easy to do and it can be done in a day or two. No check-ride and no exam required, but your EASA license and medical must be current. Basically you need to do a few hours of ground with a FAA instructor (this will cover US airspace and air law) and a check-out in the airplane, covering basic flight maneuvers (steep turns, slow flight, stalls and landings). Cost around 500$ I'd say. Check-out the link below, the official FARs (Federal Aviation Regulations) page covering issuance of a FAA PPL on the basis of a foreign license:
https://www.ecfr.gov/cgi-bin/text-id...1_175&rgn=div8
In order to get an FAA CPL/IR MEP you must have a full (stand alone) FAA PPL, since the FAA CPL/IR can't be issued on the basis of a foreign license. To get an independent (stand alone) FAA PPL you would need take and pass the PPL check-ride and written test (the written test is not that extensive as the EASA exams, studying can be done in advance at home (home-study course). All you need to prepare is the test-prep book - see link below). You will need to fly a few additional hours (~ 10hr) with an FAA instructor to cover US specific requirements for the PPL check-ride. Cost estimate approximately 2000$ to 3000$.
https://www.amazon.de/Private-Pilot-...language=pl_PL
Bu don't forget, in order to fly commercially in Europe you must convert (acquire, since it's actually not really a conversion) your FAA CPL/IR to EASA CPL/IR. This basically requires passing all 14 EASA ATP exams (don't underestimate this, since the studies are quite extensive) and you need to take the EASA CPL/IR check-ride which commonly requires 15 hr additional flight training, which must be done in Europe (usually multi-engine, since a single-engine CPL would be useless for most job opportunities). In the end you will have two independent licenses, EASA and FAA. Cost for the conversion, difficult to say, in average around 15.000 €.
2/ If I find flight school will I get a student visa? How long can i stay in the USA? Can I participle in english courses and flight courses in the same time? There are any flight school which can you recommend?
Don't know about the current visa requirements for training at a US flight school, but typically the duration of a visa for flight training is/was between 12 to 18 month (with the possibility to extend up to 24 month). But as I said, I can't comment on the current visa situation/requirements. Can't think of any flight schools offering language classes and if so, then most likely only aviation specific English. But I think this won't be necessary anyway, since you will be forced to speak English every day and there will be major improvements during your stay.
3/ I heard that a flight training in the USA is cheaper than in Europe, is it true?
Well, it depends on several factors (exchange rate, currency, not all EU member-states have the Euro, EU country) but usually it can be said that flight training is less expensive in the US as compared to Europe. But I'm not sure if that applies to Poland as well, since Poland still has the Zloty and that makes flight training in Poland quite affordable compared to other European countries. I don't know if flight training in the US is cheaper for somebody who is form Poland. I'd say that probaly depends on the exchange rate USD/Zloty and besides you need to compare the cost for flight training between polish and US schools.
4/ There is a possibility to do also a Flight Instructor licence and will obtain some experience in the USA like a flight instructor, before i will back to Europe? That's my idea for a distinction from a group of pilots with about 200h tt + CPL who are recruiting to the airlines.
Yes, certainly you can get your CFI (certified flight instructor) license while in the US, but there are a few things to consider. Adding a CFI would add to the total cost of flight training (about 5000$). How many hours could you fly as an instructor, remember your visa time is limited and depending on how long it will take you to get your CPL, there might be not that much time left. Your CFI license will (most likely) be useless once you are back in Europe and it won't help you getting a job with a European airline. It must be renewed every 24 month, but this could be done relatively easy by internet and for little money < 100$.
5/ If I arrive to the US with passed all ATPL exams then it's included and my courses are faster and cheaper? (Do I only practice?)
Be careful, don't get things mixed up here. Just for clarification, FAA license = USA and EASA license = EU! An EASA license in the USA is basically useless, and vice versa. You might benefit form your knowledge required for the EASA ATP exams, this might reduce your effort/time to study for the FAA written test/s. But that's all, it won't reduce your overall cost of training nor will it shorten your training significantly. In order to get a stand alone (independent) FAA license (PPL, IR, CPL) you still need to pass all of the required written test/s (theoretical knowledge exams) and pass the respective check-ride/s (PPL, IR and CPL). Which means you must meet the required aeronautical experience (flight training) required for the respective license. In the US there is no such thing like a conversion, except for the PPL (issued on the basis of a foreign license - see first paragraph).
Here a few US flight schools, of course there are plenty of other flight schools, you might want to do your own research! Good luck!
http://www.flyhaa.com/
https://atpflightschool.com/
http://americanflyers.com/
http://sportysacademy.com/
https://www.naples-air-center.com/flight-school/
http://www.flyeft.com/
http://epicflightacademy.com/easa-atpl-course/