In the Mishna, Megillah 1:8, Rabban Shimon ben Gamliel is quoted as having said that Greek is the only language, other than Hebrew, in which it is permissible to write sifrei Torah. Commenting on this, the Jerusalem Talmud (Megillah 71c) says that the sages checked and discovered that Greek is the only language into which it is possible to translate the Torah with its exact meaning.
But why not Aramaic? I remember reading somewhere that it had "sparks" of holiness, the proof being that it is the language of the Gemara. It's also linguistically more similar to Hebrew than Greek, so it should be possible to get a good translation.