How mellow? Like in chill-out, classical, post-rock, pop, new age, indie… ?
Anyway, I would recommend Sigur Rós, they’re from Iceland, and their vocalist sings in a made-up language they call “vonlenska”...
I personally think that ”( )” (their 2002 album) is beautiful, “Takk…” (2005) is very good too and somewhat easier to listen… last year they released Hvarf-Heim, a double recopilatory album with studio and live tracks.