Widzieliśmy go.


#1

Translation should be ‘We have seen HIM’?


#2

Not necessarily—remember that all Polish nouns have grammatical gender, and “go” is the pronoun used for all masculine nouns, whether animate or animate.

We don’t know the context here, so “go” might refer to a man (in which case it would be translated as “him”, just as you suggested), a dog or a cat (masculine nouns in Polish—translated as “it” into English) or even a car or a bicycle (masculine nouns as well).