This. ^ Going against the grain here, but I am on 'team brother.' I think for someone to learn their brother's wife's language, and then show it off at his brother and wife's wedding is really attention-seeking, and a bit insensitive. I would have been pissed off too, if I'd been the OP's brother.... It was a bit thunder-stealing.
So the OP's brother can't speak his wife's language, yeah, so what? As a few posters have said, it's not that easy for some people to learn a new language, and if the OP's brother and his new wife are living in the UK, (which it appears they are,) he doesn't need to learn her language.
My uncle married a Dutch woman who he met at work in the 1970s, they were both mid-late 20s, and she had been in the UK for 3-4 years...
Her English was very good, and she had been able to speak it for about 10 years. They got married in the UK, 3 years after meeting in the UK, and she never went back to The Netherlands. (Only to visit family once or twice a year.)
My uncle never learned to speak Dutch. He didn't need to, and his wife and family thought nothing of it. He got Married in England, and he and his Dutch wife lived in England thereafter. He had no need to speak her language. If one of his siblings had learned to speak Dutch, (purely because their brother's wife is from The Netherlands,) and gone to their wedding and spoke Dutch to the wife's family, I think everyone would have thought it very odd and attention-seeking.
@poochuspoochus YABU.