You can't talk to the enemy in the World of Warcraft. Shouting "KEK" when you're on your Alliance character doesn't read as "LOL" to the Horde players, no matter how many times you've heard that rumor. You can't Mind Control the enemy and then babble away in your own tongue (although that did work for one amazing, once-upon-a-time period in a long-ago patch). The truth is, trying to leapfrog the Alliance/Horde language barrier is a bannable offense; you're just not supposed to talk to the enemy.
That said, there is a way you can make your intentions known to players of the opposite faction: standard, pre-set emotes. The only emotes that work between factions are the ones that are already in place in the game. Creating your own emote by typing "/e yourmessage" only works for players of your own faction; for others, it translates to "X makes some strange gestures" – pretty antagonistic, really, no matter what you're actually intending to convey.