I couldn't find an official answer, so this is a combination of guesswork and theorizing, but here it goes:
First of all, not all of the spells in Harry Potter originate from Latin. For example, Alohomora is
a combination of a Hawaiian and a Latin word originates from "a West African word that meant 'friendly to thieves.'"
Therefore, witches and wizards from different parts of the world probably exchanged spells among each other. Having broomsticks, Portkeys, and being able to apparate would have allowed members of the magical community to travel to different countries, so I doubt that witches and wizards were as isolated as you would think. It would not be impossible, and as there are some spells with non-Latin roots, it is probable that witches and wizards exchanges spells that had Latin origins, and that these spells were used worldwide.
Also, I doubt that we have seen all of the spells in the Harry Potter universe, and there probably are significant amounts of spells that do not have Latin roots that we have not seen yet. That would explain why the majority of spells have Latin roots.
In addition, in the Triwizard tournament, Victor Krum cast the Cruciatus Curse on Cedric, which is a curse frequently used by British wizards. Victor Krum was from Bulgaria (which is in Europe, but it was the only reference that I could find to a foreign wizard audibly casting a spell), and while he was under the Imperius curse when that happened, it is some evidence that foreign wizards might use the same spells as British wizards.
Keep in mind, though, that there is a lot of magic that wizards can perform without using incantations, such as Occulemency, and a lot of magic performed by underage wizards who don't have access to wants (like Tom Riddle and Harry Potter). Incantations could be less popular in other parts of the world, where they could use other forms of magic.
It's not really clear how spells are created, anyway, so nobody will really know the answer to this unless JKR does another interview and explains it.