"...when I asked a saleswoman for help, she said, in perfect English I might add, "I don't speak English!" and walked off..."
i had the same thing happen about a year ago when i was at a store with my girlfriend... pretty much the same thing, she went up and asked a saleswoman a question, got the same same snot-assed "Don't speak English..." even before she had finished asking her question... (unfortunately, it had been a bad day already) And after exercising unbelievable restraint, I just looked back at her with the nicest smile i could show, and said "Wow! Well then being that this will be the furthest point you'll get in your life, I have to congratulate you for climbing to this pinnacle of your career at such a young age..."
Then, after clearly hearing her nasty insults and name-calling to me as I was leaving, I looked at my girlfriend and said "See, I knew she new English..." We still laugh about it...
You, sir, are my hero. That's priceless!
I never have a good come back in these situations, I'm queen of the way homers--the replies that come to you in the car.