Oddly enough, the only non-local I ever remember attending Sunderland matches (apart from people from South Shields, and they're pretty much still local) was a Norwegian superfan.The times I've been to Old Trafford there have definitely been people around me speaking Manc. So either there are local fans or so devoted that they learn the local accent to fit in.
But of course the majority of the fanbase comes from outside Manchester. Obviously. The same applies to all clubs with a history of success.
Mind you, I'm not trying to argue that "tourists" (spanning from actual fans from other places, to actual tourists - i.e. people with no connection to the club other than a general interest for football) don't make up a significant part of the matchday crowd. Due to several things; high ticket prices causing more locals to drop the season ticket, and sustained success in an era where traveling has become affordable for a lot of people while the general fanbase for the league has grown abroad.
At the beginning of the successful period, I'd guess most of the "tourist" fans in Old Trafford (and Anfield, for that matter) were from Ireland and Scandinavia.