I don't know about this specific extension, but if what another commenter said is right (that it's just a bookmark for play.spotify.com), then I've been able to use that site from here in Southern Europe since I found out about it (not that it wasn't a hassle to find or get going in the first place though).

I don't know why it would start out being US-only, as the native application seems to have arrived relatively late in the US.

PS: The Spotify home page is god-awful these days: a splash page plus auto-play music, I thought we as a species were over that stage...