Gemara

8b - Mishnah: Seforim are unlike Tefilin and Mezuzos in only one respect: Seforim may be written in any language, but Tefilin and Mezuzos must be written in Ashuris (Hebrew).

R. Shimon ben Gamliel says that the only other language for Seforim is Yevanis.

Contradiction (Beraisa): Kisvei ha'Kodesh are not Metamei the hands if Mikra (text that should be in Hebrew) was written in Targum, or Targum was written in Hebrew, or the letters were written Ivri (a different style of writing Hebrew letters);

They are Metamei hands only if they are written Ashuris (our character set for Hebrew), in a Sefer, and with ink. (Our Mishnah allows any language for Seforim!)

Answer #1: The Beraisa discusses Megilas Esther. The Megilah must be in Hebrew with Ashuris letters. We learn from "Ki'Chsavam vechi'Lshonam".

Rav Papa: "Pisgam of the king was heard" and "All women will give Yekar" to their husbands" are Targum in the Megilah.

Answer #2 (Rav Ashi): The Beraisa discusses other Seforim; it is like R. Yehudah:

Beraisa: Tefilin and Mezuzos may only be written Ashuris;

Raboseinu (R. Shimon ben Gamliel) permitted Yevanis.

Objection: It says "V'Hayu" (they must be in Hebrew)!

Correction: The Beraisa should say 'Raboseinu permitted only Yevanis.'

Beraisa - R. Yehudah: Raboseinu permitted Yevanis only for a Sefer Torah, because of the episode with King Tolmei. (He forced Chachamim to translate the Torah in Yevanis; miracles occurred.)

9b - R. Avahu: The Halachah follows R. Shimon ben Gamliel.

R. Yochanan: He learns from "Hash-m will Yaft (beautify) Yefes and dwell in the tents of Shem" - the words (language) of Yefes (the father of Yavan) will be in the tents of Shem.

18a - Beraisa: If speakers of Giftis, Ivris, Ilmis or Yevanis heard in their language, they were Yotzei.

Rav and Shmuel: One is Yotzei Kri'as ha'Megilah in Yevanis.

They hold like R. Shimon ben Gamliel, who permits writing Seforim in Yevanis.

Question: If so, they should have said 'the Halachah follows R. Shimon ben Gamliel!'

Answer: If so, we would have thought that they permit Yevanis for other Seforim, but not for Megilas Esther, which says "Ki'Chsavam". Therefore, they explicitly taught that one is Yotzei Kri'as ha'Megilah in Yevanis.

Shabbos 115a - Mishnah: One may save all Kisvei ha'Kodesh from a fire (on Shabbos), whether or not we read in them (on Shabbos). Even if they are written in another language, they require Genizah (one may not abandon them). We may not read in them for this draws people away from (the lecture in) the Beis Medrash.

Gemara - Rav Huna: If Kisvei ha'Kodesh were written in Targum or any other language (other than Hebrew), we may not save them from a fire;

Rav Chisda: We may save them from a fire.

Surely, the Tana who permits to read in them (for he permits Seforim in all languages) permits saving them. The Amora'im argue according to the Tana who forbids.

Rav Huna: Tana'im argue (as I and Rav Chisda do):

Beraisa: If they are written in Targum or any other language, we may save them from a fire;

R. Yosi says, we may not save them.

R. Yosi: A case occurred, and my father found R. Gamliel reading Sefer Iyov in Targum. My father told him 'Targum of Sefer Iyov was brought to your grandfather R. Gamliel (ha'Zaken), and he commanded to bury it!' R. Gamliel himself (accepted this and) commanded to bury the Sefer.

R. Yosi b'Rebbi Yehudah says, they dumped mud over it.

Objection (Rebbi): It is forbidden to overtly ruin Kisvei ha'Kodesh! Rather, we leave them in an unguarded place, and they perish by themselves.

Question: Perhaps neither Tana holds like Rav Chisda, who permits saving something that one may not read!

Answer: R. Yosi is like Rav Huna. Another Tana is like Rav Chisda:

Beraisa: If they were written in Giftis, Midis, Ivris, Ilmis or Yevanis, even though one may not read them, we may save them from a fire.

Sanhedrin 21b - Mar Zutra: Originally, the Torah was given to Yisrael in Lashon ha'Kodesh, in Ivri writing. In the days of Ezra, it was given in Arame'ic, in Ashuris writing. Yisrael chose Lashon ha'Kodesh and Ashuris; they left Arame'ic and Ivri writing for commoners.

Rav Chisda: The 'commoners' are Kusim.

Rishonim

Rif and Rosh (Shabbos 43a and 16:1): The Halachah follows a Rebbi against his Talmid. Therefore, we do not save Kisvei ha'Kodesh written in Targum or other languages, like Rav Huna, the Rebbi of Rav Chisda.

Ran (DH Gemara): The Ra'avad says that Yevanis was always permitted. Also, speakers of Giftis, Ivris... who heard the Megilah in their language are Yotzei, even though it says "Ki'Chsavam". However, this is only if they do not understand Hebrew.

Rosh (ibid.), Tosfos (115a DH Aliba), and Ba'al ha'Ma'or: This applied in the days of the Gemara. Nowadays we may read them and save them. Since (we do not understand and remember so well), there is no alternative (to prevent Torah from being forgotten), "Es La'asos la'Shem Heferu Torasecha". It was forbidden to write Torah she'Ba'al Peh, yet R. Yochanan and Reish Lakish read a Sefer of Agadata due to "Es La'asos" (Gitin 60a).

Milchamos Hash-m: Also BaHaG permits Targum. Perhaps the Rif agrees with this, and he says that we do not save Targum, i.e. other languages. "Es La'asos la'Shem" permits only Onkelus and Yonason ben Uzi'el.

Rebuttal (Ran, ibid.): Surely, "Es La'asos" permits any language people speak! Rav Sadya Gaon wrote in Arabic!

Rambam (Hilchos Shabbos 23:26): One may save Kisvei ha'Kodesh only if they are written in Lashon ha'Kodesh and Kesav Ashuris. If they are written in another writing or language one may not save them or read in them even during the week. We leave them in a place where they will perish.

Rashi (Shabbos 115a DH Te'unim); My Rebbi'im say that only Kesuvim are forbidden in other languages. Presumably this is because Yonason ben Uzi'el translated Nevi'im. This is not a proof; he did not write his Targum. The opinion that forbids allows only Chumash in Yevanis, because of the episode with Tolemy

Rambam (Hilchos Tefilin 1:19): Tefilin and Mezuzos must be written in Ashuris. They permitted Seforim to be written (other than Hebrew) only in Yevanis. Yevanis has been lost and (what people say is Yevanis) is errant. Therefore, nowadays all of these are written only Ashuris.

Hagahos Maimoniyos (70): A case occurred, a convert who did not know Hebrew wanted to copy a Nochri translation of Chumash, to use it for a Perush. R. Yo'el forbade, because the Halachah follows R. Shimon ben Gamliel. Also, we find that R. Gamliel put a Targum in Genizah. We read Targum of Chumash and Nevi'im because these were said by Onkelus and Yonason ben Uzi'el. The Kesav must be Ashuris or Yevanis. Also, '...they left Arame'ic and Ivri writing for commoners' (Sanhedrin 21b). We are like commoners.

Note: Our text says that Yisrael chose Lashon ha'Kodesh and Ashuris, and left Arame'ic and Ivri for commoners. Rav Chisda explained that the 'commoners' are Kusim. Presumably, Hagahos Maimoniyos says that we are like Kusim because many Yisraelim do not know Lashon ha'Kodesh.

Rambam (Hilchos She'ar Avos ha'Tum'ah 9:7): The Targum in Ezra and Daniel is part of Kisvei ha'Kodesh. If (text that should be in) Hebrew was written in Targum, or vice-versa, or if Kisvei ha'Kodesh was written in Ivri, it is not Metamei hands, unless it is written Ashuris, with ink, on leather.

Chazon Ish (Yadayim 8:8): The Rambam did not bother to discuss Kisvei ha'Kodesh in Yevanis, for it has been forgotten.

Poskim

Shulchan Aruch (OC 334:12): Nowadays we save all Kisvei ha'Kodesh from a fire and we may read in them no matter which language they are written in. We also Targum that was written in Hebrew, e.g. "Yegar Sahadusa", and Hebrew that was written in Targum or another language that people in the area are fluent in.

Magen Avraham (17): Granted, it was permitted to write in other languages lest Torah be forgotten. But why may we write (Hebrew) in a Kesav other than Ashuris? Perhaps it is because not everyone can write Ashuris nowadays. The Rema (Teshuvah 34) suggests that perhaps the Isur to write Torah she'Ba'al Peh is only in Ashuris, and Kesav Meshitah was developed for writing Torah she'Ba'al Peh. Certainly, we are not concerned for the Kesav of Torah she'Ba'al Peh. R. Yo'el did not allow to translate a Chumash for a convert. Perhaps he holds that "Es La'asos" applies only to something needed for everyone.