The holiday would be called "Armistice". Where from sometime late August to September 2nd there will be no world PvP. It will allow horde/alliance players to visit alliance/horde cities and allow the players to complete quests and dailies for achievements.
Breaking the language barrier for the time would be fairly cool, but certainly would not happen.
I haven't put any thought into what achievements/quests there would be so if you have an idea feel free to share.