That's when you look them in the eye, and politely, but firmly, tell them, "No. This is America and we speak English here. You need to learn English, or go live somewhere else."
Don't be polite. It's taken as a sign of weakness.
My sister is a police officer in AZ.
She has memorized several spanglish phrases(because Mexicans really don’t speak Spanish):
You will go to jail until we can find a translator.
Your child will go to foster care - it will take several weeks for the child to come home.
and so on.
I do find it intersting how good their English skills become as the cuffs go on.
My favorite:
Look Chika, just because I’m Anglo doesn’t mean I no habla. I speak Espagnol just fine - I choose to speak English because this is America. So should you. And yes, on the rare times when I visit Mexico, I use my spanglish.