Do they all speak good english?
I think these things do take a lot of time and effort to over come prejudices. I think eventually dh family got used to me asking why with things, and accepted it was just curiosity rather than being rude. Different culture, but there definitely seemed to be a similar theme of we do it just because, and it was wrong to question! It helps if you do a bit of your own research too, to show interest and give them a starting point.
I wouldn't get too uptight about them speaking to your son in their own language, it will be good for him to learn bits of it. If you keep listening to it for long enough, you will pick up bits sooner or later too! In dh family, a couple of them don't speak much english at all so it isn't fair to them neither to have all conversations in english, so there does have to be a balance. I think you need to have words with your dh though, he should be translating bits that are particularly relevant to you, and making sure you aren't isolated all of the time.
I think some cultures tend to be more wider family orientated that english culture too, so it might feel that they are a bit hands on with your son, and interfering even! You will just have to be polite and firm - but the support can be great too!
I think tbh it is your dh that is at the most fault here, for not supporting you more.