סַהֲדוּתָא עַל יְהוֹסֵף שַׁוְיֵהּ דְלָא קָרֵב לְאִתַּת רִבּוֹנֵיהּ בֵּיהּ בְּיוֹמָא נְפַק מִבֵּית אַסִירֵי וְשַׁלִיט בְּכָל אַרְעָא דְמִצְרָיִם סִפְתָא דְלָא חַכִּימַת אֲלֵיפִית שְׁמַעִית.
1. testimony | עֵדוּת – See Radak. Alternatively: "an ornament", related to the word "עדי" (opinion in Ibn Ezra, who points to such usage in Melakhim II 11:12). This accords with Bavli Rosh HaShanah 11b that the verse refers to Yosef's being freed from prison and appointed viceroy, when he was adorned with a golden chain, on Rosh HaShanah.
2. in Yosef | בִּיהוֹסֵף – This is a unique spelling of the name Yosef. Here, the name might serve as an epithet for all of Israel (Ibn Ezra and Radak), or at least the northern kingdom. Cf. Bavli Rosh HaShanah 11a and commentators in its wake that the verse refers to Yosef himself. Hashem ordained that the shofar be blown on Rosh HaShanah to commemorate his being freed on that day (see previous footnotes).
3. When He... against Egypt | בְּצֵאתוֹ עַל אֶרֶץ מִצְרָיִם – See Radak and Hoil Moshe that the clause is referring to Hashem setting the celebration of the new moon as a statute at the time of the Exodus, when He went to plague the Egyptians. Radak alternatively suggests: "when he [the nation of Israel] went forth from Egypt", explaining that here "עַל" (usually translated as "upon") is used instead of the word "מן" (from) to show that the people left "with an upraised hand". Cf. Bavli Rosh HaShanah 11b (and previous footnotes) that the clause refers to Yosef's leaving of prison to go out as viceroy over Egypt (Bereshit 41:45).
4. A language... I heard | שְׂפַת לֹא יָדַעְתִּי אֶשְׁמָע – See Radak who compares the phrase to the term "מֵעַם לֹעֵז" in Tehillim 114:1, suggesting that the speaker stands in for Israel who recall their time in Egypt when they were amidst a people who spoke a foreign tongue. Cf. Ibn Ezra that the poet might alternatively be referring to hearing the voice of God at Sinai or to listening to Moshe who had a speech impediment. Cf. Rashi and Bavli Sotah 36b that the verse speaks of Yosef's learning seventy languages. It is also possible that this phrase is a segue to the second part of the psalm, with the poet introducing the speech of God which is to follow.