There are no 'typical' sentences to please a Polish mother or a father in law. Of course politeness, respect in regard to them and their daughter is a key. Show how important and wonderful is she, and that your feelings to her are clear and serious - you are not another 'player' and you want only what's the best for her, but what's important - first ask their parents for 'permission' before you make a major move behind their back, like asking to marry her. You'll prove this way, that their acceptance in very important to you - and this is very welcomed in a traditional Polish family.
Say something about you, show your value - good education, a decent job is also much appreciated. Tell them how you can help their daughter (with studies/work/dreams), and what's your future contribution to the family.
I can help with some short translation if you want, just write something from your heart.
Best of luck!
Chciałbym powiedzieć, jak ważna dla mnie jest Państwa córka ........... (first name).
(I would like to say how important your daughter ..... is to me.
Znamy się już .... miesięcy i dobrze nam jest ze sobą.
(We have known each other for ..... months, and we find it good to be together
Bardzo ją kocham, zawsze będę ją wspierał i mam nadzieję, że ........ (first name) zostanie moją towarzyszką życia.
(I love her very much and will always stand by her and hope ......... will become my life's companion.)
I don`t think reciting such sentences you learnt by heart, most probably with the wrong accent, will bring a desired effect. Coz their content sounds quite formal while the performance will sound rather funny. It might turn into a little stand-up comedy.
