Siman 145

In the days of the sages of the Gemara, they were accustomed to translate every verse in the Torah, because they spoke the Aramaic language, and they translated so that the people would understand the explanation of the Torah. The reader is not permitted to read more than one verse to the translator. Rather, he reads one verse to him and he translates it, and afterward he reads another verse and he translates it, and so on always. And the reason appears to be: so that they do not become confused, and will understand well. The translator is not permitted to translate until the verse is completed from the mouth of the reader, nor is the reader permitted to read another verse until the translation is completed from the mouth of the translator. The reader is not permitted to assist the translator, so that they do not say the translation is written in the Torah. The reader is not permitted to raise his voice higher than the translator, nor the translator higher than the reader, as it is written: "Moses would speak, and God would answer him with a voice" – with the voice of Moses (Berakhot 45a, and see the Rif's explanation and Tosafot ibid.). (And that which we were not concerned regarding blessings that they would say "they are written in the Torah" in siman 139, is because regarding translation there is more concern since it is an explanation of the Torah. Magen Avraham subsection 2.)

One who is lesser in stature translates for one who is greater, but it is not honorable for one who is greater to translate for one who is lesser, and so it is implied in Yoma (2b). Furthermore, it is implied there: that if it occurred that the greater person was the translator for the lesser—he should finish, and another who is lesser than the first should not go up in the middle to translate, for we ascend in holiness and do not descend; see there. And now it is not the custom to translate because there is no benefit, since we do not know the Aramaic language. And if you should say we should translate in our vernacular? For one can say translation is different, as it was established with the Holy Spirit (Tur). And from this is a moral lesson against the innovators, who say regarding "twice the Scripture and once the translation" that it is not specifically the translation. And more will be explained regarding this in siman 285 with the help of Heaven. And it is no wonder: how are we able to nullify the translator that they established previously? Because even in their days there were not translators in all places, as is explained in Megillah (23b); see there. And the Tur brought the Yerushalmi, from that which we see the Rabbis go out for a fast, and they read and do not translate. This says that the translation is not indispensable. Thus far his words, and this is in Yerushalmi Megillah at the beginning of the fourth chapter. And it says there further: and even though the translation is not indispensable, nevertheless if he erred—we make him return; see there.