One of the fundamental truths in WoW is that Horde and Alliance can't communicate, as any attempt to speak comes out as gibberish, and any use of the /me custom emotes results in indecipherable motions. However, there's method to the madness of cross-faction communication, and the Project Azeroth language wiki helps to break down some of the language barrier.
For example, we all know that "lol" (spoken in Orcish) is "kek" (in Common), but did you know you can say "ee oo d a p" to a Tauren to profess your undying love? Or that the common Orcish phrase "Zug Zug" can be heard by a gnome if you say "101"? The system is designed so that very little communication can take place between the factions, but it's nice to know your enemy sometimes--and that includes their language.