Japan, you usually use "Taro Tanaka" as the goto placeholder name. Tanaka is a very common surname here and it's super easy to write as well (田中). Sato is actually the most common but also a lot harder to write (佐藤). Tanaka is also a very "working class" name, it litearlly means "in the rice field" and most likely comes from families with a background of rice farmers. Taro also a very common given name; there have been quite a few Taro Tanakas throughout history despite being the placeholder name on most forms and the like. Taro, incidentally means "Big/Strong Son".
I am not sure offhand of a female equilvalent, it's usually Taro as the placeholder.