@Kate8889
"My mom has asked them to call her by her name and not grandma but they haven't"
This is what you need to address. And as you speak the same language as the in-laws just speak up for your mum and say firmly that they have been asked multiple times and could they please just refer to your mum as X.
I also have my own native language and like you I struggled to teach my kids how to speak it because between me being at work, them being in school and then activities on weekends - I barely had enough time. But when alone in our home my (now ex DH) and I committed to speaking to them in our native language, bought books, watched videos etc etc. You can do it - it just takes effort.
You are going on and on about losing the opportunity to speak your native language and how you miss it, but you are absolutely refusing to address the unspeakable rudeness of speaking about the in laws, in front of them, in a language that they don't understand, and to complain about them.
That was incredibly rude of both you and your mum and honestly that is the bigger issue.
They obviously know you were talking about them - and not in a positive way - so they have reacted asking that English be spoken. Why should they accept being complained about to their faces in another language? You really should apologise for that and tell your Mum not to do that again. It's really not helpful for any of the relationships.
Apologise to your inlaws.
Correct them on how to address your Mum.
Teach your children your language as you wish when not with in laws.
Job done.