Per the Rules page:
Any form of diplomatic discussion between opponents must be posted in the game chat in English or in a language that all opponents understand.
As long as everyone in the game understands the language, it is fine. However, if someone does not and feels they are the target of Secret Diplomacy, feel free to file an Abuse Report in the Cheating and Abuse Forum.
here is a matter that has come to my intention for quite some games the lat few weeks, i have been playing some standard games with other dutch players and there where games among it in wich more then half off the players understood dutch (they did not all live in holland), so 80% off the players was dutch. We said something in dutch and immediatly other (english) players started to tell us that we HAD to talk english...
But if 80% off the players speak an other language then english, we could just tell that we do not speak english, just want to know, where does CC draw the line?