# have called you

Here "you" is singular and refers to Yahweh's servant. 

# I will ... set you as a covenant for the people

"I will ... make you be the mediator of a covenant with the people" 

# a light for the Gentiles

Yahweh speaks of making his servant the one who delivers the nations from bondage as if he were making him a light that shines in dark places for the Gentiles.