Prisha to Tur Orach Chaim 288:14 explains:
פי' מצלאין נוסח של ברכות הלועזים לחולים מתחיל מצלנא אנחנא כמו שמתחיל שלנו מי שבירך כו'
Matzla'in means a version of a foreign [Aramaic] blessing for sick people which begins maztlana anachna (we pray), analagous to how our prayers begin mi sheberach (He Who blessed).
For a version of this prayer, see ...
Old French was simply the vernacular language of the great -perhaps the greatest- commentator of the Torah, RASHI, who lived in Troyes, France in the 12 th century CE.
His most acknowledged skill was his capacity to explain each word, each sentence of Torah with a superior, yet unrivaled clarity.
Rav Hirsch translates (actually his grandson translated into English from the original German)
And Melchizedek King of Salem had brought out bread and wine but
he was also a priest of the most high God.
Rav Hirsch explains that this is to set him apart from the King of S'dom who came out with demands on Avram. The translation that you give above shows ...
Based on this Haamek Davar who based himself on the Shi'iltos,
the story with Malki Tzedek interrupts the story of the king of Sdom.
Passuk 17 was Melech Sdom coming out to Avraham.
Passukim 18 through 20 was Malki Tzedek with his donations and blessings.
Passuk 21 and on returns to the narrative of the king of Sdom, his offer and Avraham's rejection of it....
The Hebrew word משא literally means a 'burden', but is often used in the Prophets to mean a prophecy, and specifically a negative prophecy of a burden some nation will be made to bear.
The classical commentator Rashi explains on Jeremiah 23:33 (link is Hebrew, this is a loose translation)
'What is the burden of the Lord' - This was said as a jest, since ...