I don't think it would need this as a regular book though because I haven't bought a book that has done what you've said. I expect it to be the case in college, and that's fine. But, as for the general marketplace, I don't think people are going to even think about it, especially the average person.
This is why I don't think you should bother putting in citations. If you wanted it to be academic -- yes you should. But for what you have, no.
As far as the population of Japan, I've heard that number thrown around years by years. Wikipedia just has citation for everything, whether the writer actually put it there or not. I could go in there and add citations and so could you.
Yeah, wikipedia is a bad example, but I couldn't be bothered to go download an actual academic paper. Read those, and you'll see the same thing.
I wouldn't bother adding sources to what you have now. But I also wouldn't try to leverage this when trying to do something academic.