Euro tournament - always Northrend.
But maybe Azeroth, if both america vs america agree, they must tell gn in channel, then streamer can join game from Northrend, as i know after 1.30.2 bnet is crossrealm
btw i got informed that usa player host on Northrend will be american host, so america vs america on northrend it's okay, if america vs euro player, then always on euro host