Well, you've got to consider complicating factors, which makes this hard to measure. Those other countries aren't very affluent compared to the USA and their educational system is probably worse, plus they don't have access to the institutional infrastructure of knowledge like we do. Also, measuring rationality seems hard, etc. there's tons of problems that always pop up when we try to evaluate things like this.

I mean, I think you'd probably be mostly right and that there's not much difference in rationality between different language users, but for other reasons than the apparent average rationality of certain language-users.