Responsa for Megillah 1:8
אֵין בֵּין סְפָרִים לִתְפִלִּין וּמְזוּזוֹת אֶלָּא שֶׁהַסְּפָרִים נִכְתָּבִין בְּכָל לָשׁוֹן, וּתְפִלִּין וּמְזוּזוֹת אֵינָן נִכְתָּבוֹת אֶלָּא אַשּׁוּרִית. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, אַף בַּסְּפָרִים לֹא הִתִּירוּ שֶׁיִּכָּתְבוּ אֶלָּא יְוָנִית:
There is no difference between scrolls (of Scripture) and tefillin and mezuzoth, but that scrolls may be written in all languages [i.e., in the script and tongue of all languages], but tefillin and mezuzoth must be written only ashurith (the holy script and tongue). R. Shimon b. Gamliel says: Even with scrolls, they permitted them to be written (aside from ashurith) only in Greek. [The reason for permitting Greek above all other languages is (Genesis 9:27): "G-d will beautify Yefeth and it will dwell in the tents of Shem": The most beautiful of Yefeth; that is, the most beautiful tongue of all the sons of Yefeth, will dwell in the tents of Shem (Israel.) And there is no tongue among all the sons of Yefeth more beautiful than the Greek tongue. And the halachah is in accordance with R. Shimon b. Gamliel. However, that Greek tongue has already been lost and has become corrupted, for which reason, nowadays, we write scrolls only in the holy script and tongue.]