The Witness to The Peoples (Isaiah 55:4-5).
‘Behold, I have given him for a witness to the peoples,
A prince (nagid) and commander to the peoples.'
In Isaiah 11:10 the seed of Jesse was given as an ensign, a banner, to the peoples, to which the nations would seek. Here He is, as the Servant, given as a witness to the peoples, with the result that He becomes their prince (nagid - God-appointed leader) and commander. God's purpose originally for His Servant was that he should be a witness to the peoples (Isaiah 43:10; Isaiah 43:12), but in his Israel/Jacob form that witness failed. Now, however, the Servant in His form as the suffering Servant and Davidic king, has become that witness, and as a result receives leadership and command over them under God. The Servant and the Davidic king have become revealed as One, and His kingship will be accomplished by His witness, and not by force.
‘Behold you will call a nation that you know not,
And a nation that did not know you will run to you,
Because of Yahweh your God, and for the Holy One of Israel,
For he has glorified you.'
This may be intended as referring to the Servant, the leader and commander of the peoples. Not only will Israel be called back to God by Him, but other nations too will experience the call of God, just as Israel had at Sinai. They too will become His chosen (see Isaiah 49:6-7). And that response is due to the fact that God Himself has glorified His Servant. The singular ‘nation' may be intended to cover all nations outside Israel. Compare Psalms 18:43 where a similar thing is said of the Davidic king.
Psalms 18:43 suggests that ‘a people whom I have not known shall serve me' is a way of expressing supremacy and overlordship, and the almost instantaneous speed with which that supremacy will be obtained, ‘as soon as they hear of me they will obey me' (Psalms 18:44). It indicates one who is ‘the head of the nations' and to whom each nation will speedily and gladly come in order to learn of Yahweh (compare 1 Kings 8:41-42).
Thus here the idea seems to be that this ruler of the nations will find that as ‘unknown' nations are first contacted at the edge of His world (the Ancient Near Eastern world), so will they immediately respond to His witness, thus expressing the speed at which the Servant's message will spread and its total supremacy (compare Zechariah 8:21-23). And this will occur because Yahweh is with Him, and the Holy One of Israel glorifies Him and reveals His splendour.
The general idea does not alter if we see ‘you' as faithful Israel, for they go out in the authority of God and of the Servant as His representative.