In my Reading Buddies program at the library, there's a 9th grade girl who has been a volunteer for several years. She has a Latino last name and parents with Spanish accents, so on occasion I have asked her if she'd like to help a child from a Spanish-speaking home practice reading en espanol. Each time she shakes her head and reminds me that she herself doesn't speak Spanish.
A couple of weeks ago, a mom who doesn't speak English wanted to sign her son up for the spring session of our reading enrichment program, but I hadn't updated and printed the Spanish applications at that point. I was using my best librarian Spanish to try to tell her that I would mail her the Spanish info in a week or two (a la "Me Talk Pretty One Day," probably saying something along the lines of "I you letter the solicitation in one weeks and you me it give in desk in bookstore to put the boy in the program"), when the above-mentioned student shyly joined us, picked up the English application, and translated each question for the mom and helped her write her responses.
Afterwards, I thanked her profusely, and then asked why she had always denied her bilingualism. Here's what she said:
"My mom always makes fun of me when I speak Spanish. So I don't."