Rashi explains that the verse uses the words kiksavam (“like their writing”) and chilshonam (“like their language”) to refer to the written letters and spoken sounds of the language, respectively. The Talmud (Sanhedrin 22a) deduces from this verse proof that neither the Hebrew script nor spoken language has ever changed.
R’ Moshe Dovid Valle writes that one reason for this was for the illiterate Jews who may otherwise become incensed over the knowledge of the gentile decree, and might react violently. The idea was that the scholars who read the decree would be able to calm the restless rabble.
Furthermore, as the Talmud (Shabbos 12b) teaches, angels only understand Hebrew.
According to Rebbetzin Heller, keeping the language is an additional merit that helped rescue the Jews. As the Midrash (Vayikra Rabba 32:5) teaches, even the Jews in Mitzrayim, although they maintained next to no Jewish observance, had the merit of retaining their language. This dedication to Jewish “culture” demonstrated the people’s desire to retain a bond with their Creator.
The Yerushalmi (Megillah 2:1) learns from this verse that the public reading of Megillas Esther on Purim must be read in Hebrew. This is brought down as the Halacha (Shulchan Aruch Orach Chaim 690:8-9).
Class Participant BR suggested that the intent of this may have been to keep the secret messages and lessons of Megillas Esther hidden exclusively for the Jewish people.