Some Americans currently use "project leader" and "project manager" interchangeably even though they aren't necessarily the same. There is a recognized difference between being a "leader" and being a "manager," although it is possible to be both at the same time. To put it simply, a manager is in charge of overseeing specific tasks and day-to-day details while a leader is responsible for providing guidance and insight. I hope that makes sense to you, this is actually a difficult thing to clarify. There are numerous books and entire college courses designed solely for studying the differences between the two.
You may be able to use the terms interchangeably and properly in one situation, but in another they would refer to two seperate people. It's also possible to have one but not the other.