Google’s John Mueller posted on Google+ a heads up to those sites implementing hreflang for international support, that they must implement the rel=canonical properly or it will be as if none of the mark up is there.

Keep in mind, John would not be posting this if he did not see this come up often enough, causing issues for many webmasters.

John wrote, and this gets pretty technical:

He shared this photo to try to help webmasters understand:

If your site supports international users with various languages, check out John’s post on Google+ and this help document.

Forum discussion at Google+.