The words have the same principle and both mean an agreement, but are used in slightly different contexts. A “treaty” is used between countries to help them come to an agreement. Whereas a “settlement” is just an official agreement to resolve an issue between people. As you can see the main difference is who or what is coming to an agreement. Hope this helps!

A treaty comes into force as an attempt to end conflict or disagreement between 2 parties. A settlement is a solution to a conflict and is an end to the conflict. Hope that helps:)