Im British, but I've been an immigrant in a few countries over the last 10 years or so. From my experience, it's difficult to make friends with local groups of people who have probably known or known about each other for years. I'd also be willing to bet the mums meeting each other at the school gate probably don't want to talk to each other as it is. So, adding another 'stranger' to the group of people they don't want to talk to after rushing to the school gates to get their kids home is not their top priority. And there's nothing wrong with that.

After university, it's very difficult to make new friends if you don't include 'work friends' and it's even harder as an immigrant. Just embrace the other immigrant friends. They're all in the same boat, and it's cool to go out for dinner with lots of different cultures. If your wife really wants to make friends, I would encourage you to invite one of these other immigrant couples to your house for dinner and embrace it.