Megillah, Chapter 1, Mishnah 8. We continue the series of "ein bein" - the distinctions between different areas of halachah. Our Mishnah deals with the distinction between sacred books and tefillin and mezuzos.
The words of the Mishnah:
"Ein bein sefarim li'tefillin u'mezuzos ela she'ha'sefarim nichtavim b'chol lashon, u'tefillin u'mezuzos einan nichtavos ela Ashuris" - The only difference between sacred books and tefillin and mezuzos is that sacred books may be written in any language, while tefillin and mezuzos may be written only in Ashuris.
Sefarim - the sacred writings, that is, Torah, Nevi'im and Kesuvim, the entire Tanach. These may be written in any language, both in the script of another nation and in its language.
Tefillin and mezuzos - may be written only in Ashuris script, the script in which our Torah scrolls are written.
The opinion of Rabban Shimon ben Gamliel:
"Af b'sefarim lo hitiru she'yikasvu ela Yevanis" - Even regarding sacred books there is a limitation: they did not permit writing them in a foreign language, and Greek alone was permitted. The basis for this is the verse "Yaft Elokim l'Yefes v'yishkon b'ohalei Shem" - God shall extend Yefes, and he shall dwell in the tents of Shem. Greece descends from Yefes, and Noach blessed him that he would dwell in the tents of Shem, who are Israel, descended from Shem.
The halachah:
The Gemara establishes that the halachah follows Rabban Shimon ben Gamliel. However, the Rambam writes that this pure, original Greek has been lost to us and is no longer available, and therefore sacred books are written only in Ashuris - in the script and language we have for Torah scrolls.