It's a common enough practice that most people usually know what it means, but there's a reason you don't see flags on Wikipedia, Facebook, or Youtube. Languages are spoken in many countries, and countries are multilingual. There are quite a few articles around the web on this topic, but that's basically what they boil down to: languages are not countries. Some users may be confused or offended that their flag is not represented.
And as a Canadian I find it generally a little weird that the Canadian flag often means Canadian French, and I have to click the US flag to get English (which is of course a slightly different English than Canadian English which is probably unavailable).
I guess it's something like "language most unique to that country", no but that's not right either... I don't know.
