יְהוָ֨ה אֱלֹהֶ֜יךָ ה֣וּא ׀ עֹבֵ֣ר לְפָנֶ֗יךָ הֽוּא־יַשְׁמִ֞יד אֶת־הַגּוֹיִ֥ם הָאֵ֛לֶּה מִלְּפָנֶ֖יךָ וִֽירִשְׁתָּ֑ם יְהוֹשֻׁ֗עַ ה֚וּא עֹבֵ֣ר לְפָנֶ֔יךָ כַּאֲשֶׁ֖ר דִּבֶּ֥ר יְהוָֽה׃
L’Éternel, ton Dieu, marche lui-même devant toi; c’est lui qui anéantira ces peuples devant toi pour que tu les dépossèdes. Josué sera ton guide, comme l’Éternel l’a déclaré.
Chizkuni
כאשר דבר ה, “as the Lord had said.” This is a reference to Deuteronomy 18,15, where G-d had promised: “that He would raise a prophet amongst them, just like Moses.” He had also told him to take Joshua the son of Nun who had been filled with the spirit of G-d (Numbers 27,18).”
Sforno
ה' אלוקיך הוא עובר לפניך, so that you have no reason to be saddened by my absence. You will enjoy leadership superior to mine, by G’d Himself orchestrating the crossing of the river. 'יהושע הוא עובר לפניך כאשר דבר ה. Although, strictly speaking, Joshua is not he people’s leader in that crossing, he will be the one who nominally will be leading you across the river, just as G’d has said.
Or HaChaim
ה אלוקיך הוא עובר לפניך…הוא ישמיד "The Lord your G'd will personally cross ahead of you;…He will destroy these nations, etc." Moses had to repeat the word הוא, "He," a second time as he intended to console the people over his own death which represented a double loss for them. Firstly, it had been he who had repeatedly protected the people from G'd's anger such as at the episode of the golden calf and then again when G'd had threatened to wipe out the people after they had accepted the majority report of the spies. Secondly, Moses had defeated the two mighty kings Sichon and Og. They would therefore be concerned who would take Moses' place should the need arise.