If you're goal is to unlock the other factions, then it's easier just to follow the unlocking guide posted somewhere around here. It takes a simple change, deleting two lines from a script file.

If your goal is to pick a faction to play to learn the game quick, then both of the above suggestions - English and Spanish - work well.

If your goal is to have fun without pulling large amounts of your hair out in utter frustration, then take the Egyptians or any of the non-Catholic factions. This is because inquisitors are annoying and impossible to kill and fun-killing and annoying and game-wrecking and annoying. Did I mention annoying? They'll burn your crusade generals and factions heirs and governors at the stake. They'll rampage through your kingdom killing anything they can get their grubby little hands on. They'll ambush your army stacks, which are heading to do pious things like attack Islamic factions and decimate them before they get halfway to the goal. Whichever CA programmer dreamed this all up should be forced to listen to Barry Manilow.

Aenlic - Reformed Catholic faction player, now playing any and all non-Catholic factions with glee.