Hi,

I know nothing of the subject, but the primary assumption is wrong:
"ALL people who speak a language not shared by others in a deel tend to use that language among themselves during a transaction, anyone who does not believe this - come to New York, where there are at least 40 some languages spoken and ALWAYS in a store when price is discussed folks behind the counter use their native tongue,".

I understand Danish, which in the Netherlands is a foreign tongue, and so do some other members of my family. I don't switch to Danish with them during a deal, even though it would likely not be understood by others.
On the other hand, it's common for people in Frisia to understand both Frisian and Dutch. Which of the two languages I pick depends on which is the familiar language between the other speaker and me.

So, though secrecy may be an effect, it need not be the reason for the language choice, and chosing a language for secrecy is certainly not universal.

Bye
                                                                Mysha