I have thought about doing something like this. Not kidding.
“No habla ingles.”
Of course, if someone started with me in Spanish, I’d have to turn around and walk away.
Bottom line is it seems illegals have more rights than citizens these days.
Learn a few phrases in Esperanto, Loglan or Lojban. Being natural language derived they sound somewhat familiar, but incomprehensible.
Your odds of meeting someone who can speak any of them are pretty minuscule...