I mean, you're not wrong, but it is not really the same problem as what is being discussed here.
I do think it is genuinely difficult for native English speakers to learn foreign languages due to the global domination of English as a language of business and culture, particularly in the Western world. If you grow up in France or Belgium or the Netherlands and you want a decent professional job one day, you know that you need to learn English. And in most of these countries English as a foreign language will be a compulsory subject until you leave full time education. If you grow up in the UK or the USA or Australia and speak English as your first language, you have no way of knowing whether the most useful foreign language for you is going to be French, Spanish, German, Mandarin, Russian, Portuguese, Arabic or any number of other languages. You can start learning when you are small but unless your parents have a particular desire for you to speak a particular language and facilitate that as an extracurricular activity from a young age, any language instruction you get will be what is chosen, effectively at random, by your school. It may not be the language you wish you spoke later in life. That's not to say it isn't worthwhile, it's just that I think it's easier to see the point in learning English as a second language compared to most other languages.
What is under discussion here, in this thread, is helping children to learn more than one language as a native language. So already, the level we expect from our children is going to be much higher. I live in France and work in a professional job where most of my colleagues are not native English speakers but work in English every day. They have a high level of English by most people's standards and will all have studied it throughout their university education and used it regularly in their professional life since. But if my children grow up to speak English to the same level they speak it, I will consider that my efforts to teach them my language haven't been entirely successful. I want my kids to speak English the way native speakers speak it, the way I speak it, not the way educated professionals who speak it as a second language speak it.
My boss can negotiate a liquidated damages clause in a contract in English. But he has no knowledge of English nursery rhymes, English children's books, very limited understanding of English culture, and if he tried to speak to my mother in English and talk about her childhood memories, there would be a language barrier. I don't want there to be a language barrier between my children and my parents and extended family.
And that is what is at stake here.
The OP's parents in law might feel (unreasonably in my view) that they are being excluded from the conversation if the OP addresses her children in her language in their presence. They might worry (more reasonably, but wrongly) that their grandchildren will not learn English properly unless their mother speaks to them in English and uses English at home, and that there will be a language barrier between them and their grandchildren later on. This will not happen because they are growing up in the UK and English will be their dominant language. What might happen, if the OP's efforts to teach her children her language fail, is that her children are unable to communicate effectively with their other grandparents and extended family. The ones they already see less often because they live abroad.
Some of the people on this thread are so concerned with rudeness they they are failing to consider what is really at stake here. And it's so much more important than whether some people are offended.