The breakdown probably varies according to the time of day. In the middle of the night (for North America) I'd expect to see a higher proportion of international users, although I'd also expect the numbers aren't quite so high since HN is U.S.-centric and predominantly English only.

The number of Canadians is relatively easy to estimate though, since the geographic distribution is similar and the majority do speak English. I'd expect a roughly 8.5:1 ratio of Americans to Canadians based solely on population.