I did, however, notice a mistake in the text. Ten paragraphs in, it mentions "the document with which Pope Leo XIII, more than two centuries later, would proclaim him Doctor of the Church..."
I know for a fact that it was actually Blessed Pius IX, my favorite pope, who issued that document, which is cited (Dives in misericordia; the date given by Zenit also indicates that the document was issued during the reign of Pius). So I went and checked the issued Italian text and it says "il Documento con cui Papa Pio IX," which is correct.
However, the Vatican site also has video of the audience, and the Holy Father audibly says "Leo XIII" in Italian. So Zenit's translation is accurate, but the Pope made a slight historical mistake.
Zenit's translation is accurate
Well, that would depend on what they are translating from. If they are translating from the Vatican's Italian transcript, then their translation isn't accurate. If they create their own transcript of the Pope's live address and then translate it, then their translation is accurate.