Responsa על מגילה 1:8

We also learned in a Mishna in tractate Megilla (1:8) “There is no difference between sefarim [books of the Tanach written in holiness, on parchment, and used for personal or public study, or for reading aloud in public. Sometimes the intent is specifically Torah scrolls] and tefillin…”¹⁷⁹…..and mezuzot except that sefarim may be written in any language, while tefillin and mezuzot may only be written in ashurit [in the Hebrew language using a particular set of glyphs]. Rabban Shimon ben Gamliel says, "Even concerning sefarim, they only permitted them to be written in Greek” If that is so, then we learned in a Mishna that Tefillin cannot be written on anything but hide. One cannot deduce, as Rav Yosaif that it means only ritually pure.