Midrash Rabbah Devarim on Languages and Writing a Torah Scroll

"An Israelite who seeks to write a Torah scroll - what is the law? Is it permissible for him to write a Torah scroll in any language, or must it specifically be written in Hebrew?

"The Sages taught the following in a Mishnah: There is no difference between the laws pertaining to the writing of Scriptural Books and those pertaining to the writing of tefillin and mezuzot, except that Scriptural Books may be written in any language, whereas tefillin and mezuzot may be written only in Hebrew script. Rabban Gamliel says: The Sages did not permit even Scriptural Books to be written in any language, but only Greek. 

"What is the reason for Rabban Gamliel, who says that it is permitted to write a Torah scroll in Greek, but not in other languages? This is what our Rabbis have taught: Bar Kappara said: It is written, 'May God extend Yafet, and he will dwell in the tents of Shem' (Beresheet 9:27) - and this is interpreted to mean that the words of Shem shall be said in the language of Yafet. Therefore the Sages permitted that the Scriptural Books may be written in Greek."

-- Midrash Rabbah, Devarim 1.1