She's likely doing it as she's sad you don't want your baby to learn part of it's heritage, and I don't blame her. She may want you to get a little knowledge of language too. So you "tell" your husband not to speak his language and now you want to "tell" your MIL not to speak hers too. She probably thinks you are also rude. Have you been going on about it? You obviously don't think much of their language and heritage. You've not even thought that she likely doesn't want her son to lose his heritage either
You've "offered" to try to learn but don't sound amenable to their heritage so why would your husband now be keen to help you learn when you've only been interested in English and have mentioned that too? ie "we only speak English at home so bi-lingual no big deal"
If your ears are sullied by non-English im not sure why you'd even get with a bi-lingual man
My DCs speak both my language, and their dad (ExHs) language. I speak/understand their dad's language well enough but not brilliantly, there's still stuff they say to each other I won't understand. OH only speaks English. I say phrases to him in my language so he's started to pick it up. He's interested in learning tho.
When im chatting with family whether personally or on phone, I speak a mix of English and my language out of habit. I don't even think about it. I live in England - it happens. OH doesn't get in a tizz as he's not paranoid. He wouldn't be thinking im talking about him.
Likewise I don't bear down on DCs when they speak their dad's language it barely registers with me no way am I going to start "No! only speak English or my language". That's because Im not paranoid either so im not going to be the Language Police.
I can't imagine being with someone and not being interested in their language and culture. Id automatically want to and focus on learning it. Likewise if a man told me I must only speak English he'd have to piss off and find someone who does.
I suppose your DH & MIL.had better learn to only speak their language when you're not present. At least it will keep the peace.