Megillah, Chapter One, Mishnah Eight
1) 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.
2) Rabban Shimon ben Gamaliel says that scrolls [of the Tanakh] were permitted [by the sages] to be written only in Greek.
Section one: 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.
Section two: 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.