Many foreign words don't decline in Polish. Can be anything ranging from "aluminium" to "zombie".
It's certainly not formal, correct English. It's starting to happen a bit more in slang usage, cf. McDonalds' "I'm lovin' it".
widzę zwierzęta is using the accusative singular case of zwierzę - the plural would be zwierząt .... so I am wondering if the traduction should be "I see an animal (sing.) in the zoo"n?
No, it's not Accusative singular. For neuter nouns, the Accusative form is identical to Nominative. Same goes for 'not masculine-personal plural'. So both Nom and Acc of "animal" is "zwierzę", just as both Nom and Acc of "animals" is "zwierzęta".
"zwierząt" is Genitive plural.
Yes. The context would probably be rather limited, like describing a picture, but it works.
P.S. It's more probable to say "Czy mogę powiedzieć" - to say it at some specific occasion, not to 'be saying' it generally.