Translation:Let's be kind.
"Polite" is correct btw.
It usually translates to "uprzejmy", but yeah, you're right. Added.
Yup, well done!
What is the dictionary form of badzmy?
The basic one... well, "być". That's just a form of "to be".
https://en.wiktionary.org/wiki/by%C4%87#Conjugation - imperative, 1st person plural.