Let’s take the Rhine Which goes from Switzerland to Netherlands. When and how did people start agreeing on giving-it the same name ? What about the alps going from southern France to Austria ? When did people agreed that it was the same mountain range ? And obviously what about the Gulf of Mexico ?
I recognize that you are asking about European place names but how humans named things in North America might give some insight. A really great book on how places got their names in the US is Names on the Land: a historical account of place-naming in the United States by George R. Stewart (ISBN 978-1-59017-273-5).