John Wesley on Psalms 81:5

Joseph - Among the people of Israel. Testimony - For a witness of that glorious deliverance. He - God. Went - As a captain at the head of his people. Egypt - To execute his judgments upon that land. I - My progenitors, for all the successive generations of Israel make one body, and are sometimes spoken of as one person. A language - The Egyptian language, which at first was unknown to the Israelites, Gen 42:13, and probably continued so for some considerable time, because they were much separated both in place and conversation from the Egyptians.