Sometimes I think some of my ILs don't get it. But their confusion is different from your parents'. They think it's a good idea that dd learn English, but they don't quite get that I need to speak it to her for that to happen. They thought it was odd when that I started speaking English to her right when she was born. When did they think I was going to start? Boh.
Some of them wonder how she'll learn Italian if I speak English to her. Seriously. We live in Italy and they're worried that she won't learn Italian! It's as if they think I'm hard-wiring her brain for English and I'm ruining it for Italian. They don't get that she will be fluent in both. But none of them is bilingual, so I think it may just be a hard concept for them.
Now that dd has begun to talk, I notice that her English words seem not to matter with some people. Yesterday my husband's aunt was over and was telling dd not to touch something because it was hot. Dd had actually already noticed it was hot, and even told the aunt that, but in English. So I told her, she knows it's hot, that's actually what she just said, and don't worry, she won't touch it because she's afraid of hot things. But the aunt just kept telling her not to touch it because it was hot. (She wasn't doing it to teach her the Italian word, just to keep her from touching it.) I don't know, maybe she just didn't believe me? But other times this person has acted as though dd wasn't talking yet even though we had told her she was saying some English words.
Dd is only 14 mos. and just beginning to talk, so I'm very curious to see how it will develop.
Maybe in time your parents will begin to understand.
BTW, the Spanish thing made me laugh. The first time my uncle met my dh, he kept trying to speak Spanish to him!