I once read that a Greek Sefer Torah is Kosher, is this correct? Is it also correct that Greek is the only language other than Hebrew that a kosher Sefer Torah can be written in?
In the body of your question you ask:
Is a Greek Sefer Torah Kosher?
Is Greek the only language other than Hebrew that a kosher Sefer Torah can be written in?
The answers to these questions are to be found in the Rambam’s Mishneh Torah, Laws of Tefillin, Mezuzah and Sefer Torah Perek 1 Halacha 19:
Tefillin and mezuzot may be written only in Assyrian script. Permission was granted to write Torah scrolls in Greek as well. That Greek language has, however, been forgotten from the world. It has been confused and has sunk into oblivion. Therefore, at present, all three sacred articles may be written using Assyrian script alone.
The title of your question is: Why is a Greek Torah Scroll Kosher? The reason is given in the Gemoro Megilla 9b
R. SIMEON B. GAMALIEL SAYS THAT BOOKS [OF THE SCRIPTURE] ALSO ARE PERMITTED TO BE WRITTEN ONLY IN GREEK. R. Abbahu said in the name of R. Johanan: The Halachah follows R. Simeon b. Gamaliel. R. Johanan further said: What is the reason of R. Simeon b. Gamaliel? Scripture says, God enlarge Japheth, and he shall dwell in the tents of Shem;(1) [this means] that the words of Japheth (2) shall be in the tents of Shem.
and the words of Japheth in the tents of Shem take the form of a Sefer Torah written in Greek.
(1) The Jewish people are descended from Shem.
(2) Javan (Greece) is reckoned among the sons of Japheth in Gen. X, 2.