Charles Ellicott Commentary Psalms 81:5

"He appointed it in Joseph for a testimony, When he went out over the land of Egypt, [Where] I heard a language that I knew not." — Psalms 81:5 (ASV)

Joseph. — The prominence given to this name indicates, according to some critics, that the author belonged to the northern kingdom. But when a poet was wishing to vary his style of speaking of the whole people—the names Israel and Jacob have just been used—the name Joseph would naturally occur, especially with the mention of Egypt, where that patriarch had played such a conspicuous part.

Through the land of Egypt. — The Hebrew means either upon, over, or against, but none of these meanings will suit with Israel as the subject of the verb. Therefore, the Septuagint, disregarding common usage, renders “out of Egypt.” But God is doubtless the subject of the verb, and we may render, over the land of Egypt, in allusion to Exodus 12:23, or against the land of Egypt, in reference to the Divine hostility to Pharaoh.

Where I heard ... — The insertion of the relative adverb, where, makes this refer to the Egyptian tongue , giving an equivalent for, “when I was in a foreign country.” So apparently the Septuagint and Vulgate. But the expression, words unknown to me I heard, when followed by an apparent quotation, most naturally introduces that quotation. The poet hears a message, which is borne to him on the festival music, and this he goes on to deliver.