Clandestino your post is really aggressive. Did you mean it to come across in this way?
I am not sure why cultural expectations here are being landed at the door of a two year old.
But as you ask, I would dress smart for a wedding, unless told not to. I did wear very dark blue to a wedding once as I was pregnant, huge, and had very little else to wear that was smart. But generally I would avoid black at a wedding (or white) and would follow guidance at a funeral.
I dislike being told that everyone has to wear a set thing. I dislike being told my kids have to wear a set thing. Maybe the OP feels like that. Maybe it's not specifically about a kilt, but even if it is, so what. She clearly loves someone Scottish.
So, I find the idea of saying every male at a wedding must wear a kilt very prescriptive, and although I may think it was great to dress my son up in a kilt, he may not be happy to wear one. Getting him to put on a shirt instead of a T-shirt for a nice meal was a struggle. And I can remember struggling with my dd at age 4 to wear anything smart to a wedding at all.
So I think a bride and groom being very offended that a small child is not wearing a kilt is very OTT.
And if I were the OP I'd probably leave the details of my son's wedding outfit up to his dad.
But even if she does that, it doesn't mean she "...doesn't give a fuck about the cultural traditions of her husband's nation and wants to make it shown?"
And to take it as such seems very OTT to me.