I find it braggy and quite honestly I worry about being judged by them as my child can barely cut up their dinner! that much is clear, everything you've said is blatantly from a place of deep insecurity. There really is no need to try and tear others down to feel less insecure/less accomplished/less than the other mums.
You wouldn’t introduce yourself in a new job by saying “Hi, I’m Sarah and you may hear me speaking Russian at all times as I like to immerse myself 100% into a new language"
In your OP you said the mum said you may hear them speaking Russian as he's learning it. That's a perfectly fine thing to say and a bit different from starting a new job and saying you'll only be speaking Russian, come on now.
Likewise, if I was Sophia’s mum I probably would have written...
You're not Sophia's mum. People do things differently. If this did happen then people are just sharing trivia about their kids, which is a bit more interesting than 'my kid is named Sophia and she's four', which could be said of every parent (changing the name of course).
The way you would do something doesn't have to be the way others do it. I think, in the nicest way possible, it might be worth having a think about your self esteem and competitiveness and insecurity re your child's progress before he begins, as this attitude will be apparent a mile off and just give the impression you have a chip on your shoulder and are sneering at everyone else whose kids have any kind of skill or talent you deem unusual.