WPML support

It doesn’t, no, but it should be possible to do that with some custom code. The bgmp_get-map-placemarks-query controls which posts are pulled from the database.

I don’t understand why core.php would need to change; I would think it’s possible to alter the query to pull the posts assigned to the current language. If not, why not?

There’s also a few other filters in that function that you could use, but I think the most performant way would be to alter the query.