> One thing I've never seen discussed on this topic (possible I just missed it, I only read popular accounts) is whether speaking multiple languages is a proxy for higher sociability / stronger social ties.

Yes. This is exactly what you should be asking with this kind of stuff. The research is hopelessly confounded by social status traits that correlate with wealth.

And before anyone says it, the abstract claiming "adjustment for linguistic, physical, and sociopolitical exposome factors" is fine, but it's essentially impossible to control for something as pervasive as the effects of wealth, without randomization. There are also factors -- like the culture you grew up in -- that are equally difficult to control. For example, if your dataset has only X multi-lingual Americans, and 1000x multi-lingual Western Europeans, no amount of statistical massage will correct for the imbalance.