They've released plenty of albums where they sing in Icelandic. The fact that they chose to experiment with something else once doesn't really take too much away. Besides, while from another country, Sigur Ros' music isn't particularly foreign; it borrows from a lot of concrete examples of post-rock/arabiance, and I don't think the fact that they don't sing in their native tongue on occasion does much.
The album that Hopelandic appears on (part of