Where is it mentioned in the Old Testament that the Messiah’s promised coming is for all people, not just the chosen few of Israel?
Here are a few.
Ps. 22:27 All the ends of the earth shall remember and turn to the LORD; and all the families of the nations shall worship before him.
Is. 42:6* "I am the LORD, I have called you in righteousness, I have taken you by the hand and kept you; I have given you as a covenant to the people, a light to the nations,
Is. 60: Arise, shine; for your light has come, and the glory of the LORD has risen upon you. 2 For behold, darkness shall cover the earth, and thick darkness the peoples; but the LORD will arise upon you, and his glory will be seen upon you. 3 And nations shall come to your light, and kings to the brightness of your rising. 4 Lift up your eyes round about, and see; they all gather together, they come to you; your sons shall come from far, and your daughters shall be carried in the arms.
Mal. 1:11 For from the rising of the sun to its setting my name is great among the nations, and in every place incense is offered to my name, and a pure offering; for my name is great among the nations, says the LORD of hosts
Joel 2:28*
"And it shall come to pass afterward, that I will pour out my spirit on all flesh; your sons and your daughters shall prophesy, your old men shall dream dreams, and your young men shall see visions. 29 Even upon the menservants and maidservants in those days, I will pour out my spirit. 30 "And I will give portents in the heavens and on the earth, blood and fire and columns of smoke. 31 The sun shall be turned to darkness, and the moon to blood, before the great and terrible day of the LORD comes. 32* And it shall come to pass that all who call upon the name of the LORD shall be delivered; for in Mount Zion and in Jerusalem there shall be those who escape, as the LORD has said, and among the survivors shall be those whom the LORD calls.