Hrvatski Noahid: Seven Gates of Righteous Knowledge, p 175 (in the pdf edition and 181 in the printed edition) explains clear language as a clear conception of truth and Godliness. A Gentile is permitted to learn from books in a language he understands.
Rabbi Smith: I didn't forget about this and looked in the original Tanach with the commentaries and found as follows - almost all the commentaries (5 out of 6) printed in the Mikros Gedolos specifically state that the "clear language" is Loshon HaKodesh, the Holy Tongue, ie. the language of the Torah, and that the nations will speak this language. The Metzudos Dovid goes so far as to say the nations will speak Loshon HaKodesh in the place of their current languages. The Radak says that the clear language means that the nations will no longer mention other gods, but then he proceeds to quote the Ibn Ezra who says it is referring to the nations speaking Loshon Hakodesh. The Malbim says the clarity of language means that the non-Jews will no longer mix in false beliefs into their speech. But the Malbim's emphasis on the content of speech does not contradict the others' explanation that the clear language refers to the language of speech. I am curious to hear why the Seven Gates of Righteous Knowledge only shares the emphasis of the Malbim.
Hrvatski Noahid: Thanks for sharing these commentaries on Zephaniah 3:9. We are not allowed to derive Torah law from the prophecy of anyone other than Moses. As a matter of practical Torah law, a Gentile is permitted to learn from books in a language he understands (the Divine Code by Rabbi Moshe Weiner, 2nd edition, p 87-88).
Rabbi Smith: To clarify, Zephaniah 3:9 is not saying that a non-Jew today cannot learn Torah in the language he understands. Not only did Moshe Rabbeinu write the Torah in 70 languages, but the Lubavitcher Rebbe said the reason that his father-in-law sought to have Toras HaChassidus translated into other languages was so that non-Jews could learn Toras HaChassidus. But, Zephaniah 3:9 is prophesizing that the all the people in the world will come to know Loshon Hakodesh, the Holy Language.
Hrvatski Noahid: According to Rabbi Weiner and Dr. Schulman, a clear language means a clear conception of truth and Godliness. This explanation is in accordance with their ruling that Gentiles are commanded to know God as a positive aspect of the Noahide prohibition of idolatry. It is prohibited for a Jew to teach Torah to Gentiles in the canonized Hebrew or Aramaic text (see Tractate Hagigah 13).