> So I say to Facebook and Google, "use the language of the user, as defined in their browser language s settings primarily, but offer the user a choice to change it. I'd like 25% of the increased sales"

Unfortunately this is not always accurate. Chrome on my Mac for example uses "en-us" despite the fact that my machine is entirely configured to use British English (en-gb). For my own websites I'm forced to geoip the location to infer which subset of English to use - most annoying.