“Who among you fears Yahweh.” Isaiah 50:1-3 was God speaking, Isaiah 50:4-9 was the Messiah speaking, and now Isaiah 50:10-11 is Yahweh speaking again.
Isaiah 50:10 is translated a little differently in the English versions, primarily because it involves an ellipsis. A quite literal translation of the Hebrew text would be: “Who among you fears Yahweh, hearing the voice of his servant” with “hearing” having the meaning of obeying. If we complete the ellipsis, the reading becomes something like, “Who among you fears Yahweh? Who obeys the voice of His Servant?” The two lines together make the point that the person who fears Yahweh will obey the voice of His Servant, the Messiah.