It seems a little weird to me that Germany and Japan call themselves Deutchland and Nippon.
In English these names come from all over the place. In Germany the name comes from the Latin Germania used to refer to the area north of the Danube, coming from the Germani tribe.^^1
In general exonyms will have multiple different answers.
^^1 Hagen Schulze, Germany: A New History (Cambridge: Harvard University Press, 1998), 3-5.