Megillas Sesarim writes that the word “pur,” coming from the foreign language of the Persian nation that had dominion over the Jews, has a negative connotation. Had the verse used the word “goral,” it would have had a positive connotation – especially with its allusions to the Yom Kippur service. Since the lot decided when the ideal time to massacre the Jews, the verse uses the Persian word.
On the other hand, since the lots also helped precipitate Haman’s downfall, the verse uses the Hebrew word, as well. In a deeper answer, the Pachad Yitzchak writes that the Jewish people always had two kinds of adversaries in their history – the four nations in which they were exiled, and the seven Canaanite nations they had to eject upon entry to the Holy Land. Therefore, he writes, “pur” is translated to emphasize the dual nature of Haman, in that he was both (as an Amalekite) from the seven nations in Israel and (as a Persian officer) from the four nations of exile.