Translation:I do not remember this party at all.
In English there's a difference between "I don't remember at all" (0%) and "I don't completely/entirely remember" (I remember some of it, but not 100%). Which meaning does 'zupełnie' have?
"Zupełnie" is nowadays used mostly in negative sentences (like this one) and it then means "absolutely nothing". On its own it means "absolutely full/all".
Then it should definitely be "that party". Something that you can't even remember can't be "this" (perhaps an exception would be in "A Christmas Carol" when a ghost transports you to the past).
And that creates a difficulty. "that" is of course accepted, but if we used "that" in the default answer, we have to also make "tamtej" the default Polish one. Which is perfectly possible, but not necessarily more natural. Similar with the 'dog' sentence where you also commented. Sometimes you just cannot be perfectly natural in both languages, so we have to go with Polish then.
I agree. You do the best you can given the idiosyncrasies of the languages. If I point out something I completely understand that it might not be possible to fix.
Keep up the good work and soon I will be able to comment in Polish!
The meaning may be close, but then you completely changed the tense and even the verb...
I used to confused about why the "zupełnie" starts the sentence, but I now think of it as emphasizing the entirety of "nie pamiętam tej imprezy".
Like that cheese sentence that says "zupełnie zielony", but instead of one word "zupełnie" emphasizes an entire sentence.