"The party is in the garden."
Translation:Impreza jest w ogrodzie.
"Przyjęcie". It kinda depends on what kind of a party it is. "Impreza" is more likely to be all about fun, and when the party-goers are not kids - probably about alcohol as well. "Przyjęcie" allows for the party to be more fancy and elegant. I would not call any elegant charity ball "impreza".
For a birthday party of a kid, I don't see much difference.
"Zabawa" sounds a bit old-fashioned to me, as it's totally vague, it comes from the verb "bawić się", to play. So it doesn't say a lot. This is something my grandma could use when talking about her youth. It may not even be a party, just some friends doing something for fun, like dancing, or playing board games. It's hard to say what can be 'incorporated' under this term.
"Balanga", on the other hand, sounds totally like a crazy party, and feels very colloquial to me. Well, "impreza" is already colloquial I guess, but this is more.