In the basic form, lightning-network is a chain of iou:s in one currency. But as far as I have understood, the plan is to put atomic swaps on lightning network. Will this mean that all currencies exist in the same lightning network? Does LN care and know about the curreny that is exchanged? Will it also work out the routing? What about exchange rates?
The Lightning Network can be used across currencies, but each individual payment channel must only correspond to one currency. Furthermore, each currency must use the same inputs and outputs system that Bitcoin uses and a similar scripting system for the HTLCs to work. Other than that, I think the Lightning Network should work across multiple currencies.
What about exchange rates?
Exchange rates don't matter to anything technically, so they have no effect on LN.