While the U.S. doesn't have a standard, primary language... I find the use of non-English in the US is a way for a group to cordon themselves off. It sets up an intentional wall in a manner similar to installing a fence around your yard.
While language may be foreign... tone, body language, aren't.
I found myself paying attention to the conversation I didn't understand. I was riding along with the tone. And found myself chuckling at a pause point that felt like a humourous statement.
I found the one woman unnerved that I chuckled with her friend. It was as if I'd walked through her gate. She seemed to be wondering. "Oh, crap. Does he speak my language?"
So not eavesdropping. But a very interesting sociological and psychological experiment.