אַתָּ֣ה תְדַבֵּ֔ר אֵ֖ת כָּל־אֲשֶׁ֣ר אֲצַוֶּ֑ךָּ וְאַהֲרֹ֤ן אָחִ֙יךָ֙ יְדַבֵּ֣ר אֶל־פַּרְעֹ֔ה וְשִׁלַּ֥ח אֶת־בְּנֵֽי־יִשְׂרָאֵ֖ל מֵאַרְצֽוֹ׃
דו װעסט רעדן אַלץ װאָס איך װעל דיר באַפעלן, און אַהרן דײַן ברודער װעט רעדן צו פַּרעהן, ער זאָל אַװעקלאָזן די קינדער פֿון ישׂראל פֿון זײַן לאַנד.
Rashi
אתה תדבר THOU SHALT SPEAK once, every separate message just as you have heard it from My mouth, and your brother Aaron shall express it in eloquent language and explain it in Pharaoh’s hearing.
Ibn Ezra
You will speak to Aharon, and Aharon, your brother, will speak to Pharaoh. Although Aharon is not mentioned every time Moshe approached Pharaoh, the Torah informs us here that they always appeared together.
Rashbam
ושלח, he should tell Aaron to tell Pharaoh to let the Israelites go.