"They are people and we are people!"
Translation:One są ludźmi i my jesteśmy ludźmi!
"One sa ludzmi i my jestesmy ludzmi"
Why is 'my' needed in this sentence? Does the meaning change if it is left out?
This sentence emphasizes how 'They' and 'We' are the same. It would sound very strange if you didn't include the second pronoun. In fact, if you change the subject in the middle of the sentence, you rather should put it explicitly.
Is there anything wrong with using "oni" instead of "one" in this case?
In this case I think it should be "oni", people is a mixed group, to use "one" it should then have been "kobietami" instead of "ludźmi".
Usally - not really. You rather omit the pronoun because in most contexts it's totally redundant, so you just choose it to emphasize or to contrast it with something else, mostly when changing the subject during the sentence.
Here, maybe there's no contrast, but there's the change of the subject. So without "my", it would feel quite strange.
How do I know when to use ”są" and when to use "jesteśmy"?
I'm sure I'm being stupid here, but I just cant figure these out.