Beyond watchwords, there's the language barrier. If you're supposed to be helping the Romans kill Gauls, odds are the people yelling in what seems to be Latin are on your side, and the guys yelling in some Gallic language are on the other side. Problems start to arise if you're a "friendly" Gaul, of course, since the Romans will have trouble telling you apart from the foe. Of course, that's an old problem--during the Peloponnesian war, the Athenians had trouble distinguishing their Dorian allies from their Dorian enemies, because they both sung their battle hymns in Dorian.