Isbah from Thunderlord asks over on the Priest forums if it's possible to communicate with the opposing side. The answer? It is, but it's not easy. By now, everyone knows that if Alliance hears a Horde player saying "kek," they're really saying "lol." But it's also possible to go the other way-- if you do it carefully, you can say what sounds like nonsense to you, and the opposing faction will hear words in English.
Most of the solutions players present to Isbah come from the great Project Azeroth (we've covered it here before), a "Language Annex" that's compiling a list of what works crosslanguage and what doesn't. But a few of the players in the thread offer combinations that even Project Azeroth hasn't documented yet.
Robble says that if you're Horde, you can say "qq" and it will come out as "ha" (which is really funny). "ok" appears as "no" to Alliance when Horde says it (very useful), and "ok ivx" comes out as "no kil" (if you're trying to avoid a murder, I guess).
And Relic from Balnazzar has even better combinations, built from Project Azeroth phrases:
What Alliance Says = What Horde Hears
"go as sd d a p" = "me love you"
"d a p yu vb" = "you lose"
"d a p go a zz fff bb a zz" = "you me one verse one"
"p bb go ee fff" = "use me lover"
"you lose" and "you me one verse one" might come in pretty handy on the battlefield, but "me love you"? Too much of this cross-faction talking, and the war in Azeroth might come to an end!
*By the way, I'm the gnome in the picture above. Anyone want to figure out what the big cow was saying to me? It's "kek maza kaz" if the picture is too small.