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.
The emotes you're looking for are the ones that involve any combination of set text, vocalization or character animation. While that may sound skimpy and potentially disappointing, you'll actually discover a fairly rich vocabulary.
Want to combine forces with an enemy player at a quest spawn? Look how clearly you can communicate your intentions with emotes.
/greet You greet <Enemy Player> warmly.
/introduce You introduce yourself to <Enemy Player>.
/talk You want to talk things over with <Enemy Player>.
/point You point at <Quest Target>.
/helpme You cry out for help!
/work You work with <Enemy Player>.
/volunteer You look at <Enemy Player> and raise your hand.
/knuckles You crack your knuckles while staring at <Quest Target>.
/ready You let <Enemy Player> know that you are ready!
/followme You motion for <Enemy Player> to follow.
/victory You bask in the glow of victory with <Enemy Player>.
/commend You commend <Enemy Player> on a job well done.
/salute You salute <Enemy Player> with respect.
/thank You thank <Enemy Player>.
/bye You wave goodbye to <Enemy Player>. Farewell!
Obviously, you don't need to get that long and involved to get your point across. The more emotes you make, however, the better your chance that the other player will realize that your gestures are not random and stop to listen to your message. Whether you come in peace or are seeking a more intimate connection with the object of your destructive affections, standard emotes help you make the statement.
WoW Rookie feeds you the basics to get you off to a good start in the World of Warcraft, from game lingo to joining your first guild and even what to do when you finally hit level 80.