Is it ok to greet his family and friends with a kiss on a cheek the first time? or is it weird?
I'd say this is rather reserved for family and close friends, so I'd probably wait with that if I were you - Poland is not France :)) You could try to give his family members a hug though, I guess. In case of friends I'd just give them a handshake - I'd say this is a safe choice for a first meeting (unless they're huggy people, then they'll hug you first).
Poles are often pretty reserved people, we aren't very expressive, at least in comparison to Latin people, but they get warmer once they get to know you.
it would be disrespectful if I suggest to go dancing with his friends? or how can I gain their approval?
Your boyfriend is going to be your host, so it's on him to organise everything and make sure you'll have a good time with his friends, imho. Just talk to him about what you'd like to do.
I read about Poland history, gastronomy (but I don't know how to make any Polish dish) and culture.
That's nice, Poles like it when foreigners know something about their country or at least show interest - I'm sure you'll score points with that :)
As for his mother - a way to win her over could be eating whatever she gives you to eat and say how tasty everything is lol ;)
As for language barrier - just ask your boyfriend to translate for you and maybe you could use Google Translate or some app.
Don't worry, Poles know how difficult their language is ;D