Why not just step a distance away and whisper? Private conversations don't need to be had in other languages in front of people to be private - I would venture to say that IC, that is actually a pretty good way to risk your conversation in the first place, as you have no way of knowing who speaks something or not. Not to mention it could get you a punch in the face if people start getting ticked off at being IC excluded when they're standing right there. Right now it just kind of sounds like "I understand the reasoning there, but I want to be able to exclude people to their face like other people do to me," which seems like not exactly a solution to anything. If publicly excluding others is the IC goal though, you could always devise codes in character and use them among others. Why not?
That's pretty much the thing about humans. They don't
have any one racial language or culture or style or anything. Elves may have different factions and subraces and beliefs, but in the end they are all still elves and speak a common tongue - their generations are so long that it would take far, far longer to split off into unique individual pockets. Dwarves live for centuries longer as well - even a halfling lifespan can nearly quadruple that of a humans. These all have racial languages - even sea elves, long split from land elves, may still speak elven, if an odd, accented dialect.
Humans just don't have a common language. There is no "human" tongue other than common. If you wanted to make some equivalent to 'old racial tongues' of humans, you'd need to make dozens at least, heh, to account for all the many different crazy cultures humans have created over the years and the many tongues they spoke...

It would be fun if a non-common speaker got plaintext garbled though, hehe. I don't think th - actually now that I think about it, some of the new chat functions might actually be able to handle that in NWN's future, but still...likely a project for another time and place!