Siman 145
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א בִּימֵי חַכְמֵי הַגְּמָרָא הָיוּ נוֹהֲגִים לְתַרְגֵּם, כְּדֵי שֶׁיָּבִינוּ הָעָם. אֵין הַקּוֹרֵא רַשַּׁאי לִקְרוֹת לַתֻרְגְּמָן יוֹתֵר מִפָּסוּק אֶחָד, וְאֵין הַמְּתַרְגֵּם רַשַּׁאי לְתַרְגֵּם עַד שֶׁיִּכְלֶה הַפָּסוּק מִפִּי הַקּוֹרֵא; וְאֵין הַקּוֹרֵא רַשַּׁאי לִקְרוֹת פָּסוּק אַחֵר, עַד שֶׁיִּכְלֶה הַתַּרְגּוּם מִפִּי הַמְּתַרְגֵּם; וְאֵין הַקּוֹרֵא רַשַּׁאי לְהַגְבִּיהַּ קוֹלוֹ יוֹתֵר מֵהַמְתַרְגֵּם, וְלֹא הַמְּתַרְגֵּם יוֹתֵר מֵהַקּוֹרֵא; וְאֵין הַקּוֹרֵא רַשַּׁאי לְסַיַּע לַמְּתַרְגֵּם, שֶׁלֹּא יֹאמְרוּ: תַּרְגּוּם כָּתוּב בַּתּוֹרָה.
1 The Laws Of The Translators. Containing 3 S'ifim: In the days of the Sages of the Talmud, they practiced translating [the reading into Aramaic when publicly reading] in order that the people would understand. The Torah reader is not permitted to read for the translator more than one verse, and the translator is not permitted to translate until the verse is finished by the Torah reader. The Torah reader is not permitted to read another verse until the translator has finished the translation. The Torah reader is not permitted to raise their voice louder than the translator, nor the translator louder than the Torah reader. And the Torah reader is not permitted to assist the translator, so that [people] won't say that the translation is written in the Torah.
ב קָטָן מְתַרְגֵּם עַל יְדֵי גָּדוֹל, אֲבָל אֵינוֹ כָּבוֹד לַגָּדוֹל שֶׁיְּתַרְגֵּם עַל יְדֵי קָטָן.
2 A minor may translate on behalf of an adult, but it is not respectful for an adult to translate on behalf of a minor.
ג הָאִדָּנָא לֹא נָהֲגוּ לְתַרְגֵּם, מִשּׁוּם דְּמַה תּוֹעֶלֶת בַּתַּרְגּוּם כֵּיוָן שֶׁאֵין מְבִינִים אוֹתוֹ.
3 Nowadays, our custom is to not translate [aloud from the Aramaic Targum during the public reading], for what use would it be since no one would understand it?