I once started speaking French and the student answered back in French.
I agree with the squirrel that Danish is a great choice because who the hell would bother to learn Danish?
When someone approaches and says hello, i'm friendly and greet them back. After that for everything they say, i tell them i don't understand.
Student: Hello
Me. : Hello
Student: What's your name?
Me. : I'm sorry, I don't understand you.
The look of dejection on their faces is wonderful.
Sometimes I'll use a Trinidad accent and slangs which they have no chance of deciphering.
A couple of times I tried speaking Pig Latin and a few times just gibberish.
With the babes I'll just speak Chinese.
But by far my favourite is 'i don't understand you' crushing their confidence so that they feel the need to take private classes and give all of us more jobs..
You're welcome.