פירוש על מגילה 1:8

נכתבין בכל לשון – In the writing of every nation and in the language of every nation.

There is no difference between scrolls [of the Tanakh] and tefillin and mezuzahs except that scrolls may be written in any language whereas tefillin and mezuzahs may be written only in Assyrian. Scrolls of the Tanakh may be written in any language and in any type of writing. However, mezuzot and tefillin may be written only in Assyrian, the alphabet in which Hebrew was and is still written and they may be written only in Hebrew.

אין נכתבין אלא יונית – And the reason that they permitted scrolls in Greek more than other language, as it says in the Bible (Genesis 9:27): “May God enlarge Japheth, and let him dwell in the tents of Shem…” The beauty of Japheth, that is, the beautiful language of Japheth [that is found] among all of the children of Japheth, “let him dwell in the tents of Shem,” and there is no more beautiful language among all of the children of Japheth like the Greek language. And the Halakha is like Rabban Shimon ben Gamaliel. However, at this time, that Greek language has become lost and confounded. Therefore, at this time, we do not write scrolls other than in the Holy Writing and in the Holy Language.

Rabban Shimon ben Gamaliel says that scrolls [of the Tanakh] were permitted [by the sages] to be written only in Greek. Rabban Shimon ben Gamaliel says that while Tanakh scrolls may indeed be written in languages other than Hebrew, they may not be written in any language, just Greek. At the time of the Mishnah Greek was the international language of the intelligentsia. It was also the language into which the Tanakh had already been translated. This translation is called the Septuagint and was widely used in the period by Jews in the Greek-speaking Diaspora.