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, this does not mean that Bangala, Cantonese, Gaelic, Russian, Ukrainian, Finnish, Norweegian, Vietnamese, Korean, Macidonian, Romaness, Rumaninan, Italian, Hungarian, Japanese, Croatian... etc... are "secret" languages or "cant"

What you often hear with minority languages is something different: the minority language isn't used for the actual transaction, but as a kind of assertion of identity. Something I've heard in shops in Turkish Kurdistan: the customer and shopkeeper exchange greetings and pleasantries in Kurdish, then switch to Turkish to do the purchase. I heard similar interactions using Maaori when I was in New Zealand.