In Poland typically, both officially and informally, one says N. N. (Latin nomen nescio, identity unknown).

In Arabic its “فلان الفلاني" “fulan al-fulany”, which roughly translates to “someone from some family”

Hm, well about 40% of our population is named Nguyễn. I don't think there's an equivalent expression, but I'd choose Anh Nguyễn. In addition to being a name for both genders, 'Anh' just means something like 'sir' or 'older brother'.

In Spain we use "Fulano de Tal". It seems Fulano comes from the arabic language, meaning "Anybody". We use it when we don't the name of a person we are talking about, or we really don't care to know the name. Apart of Fulano, we also use Mengano.

Fulano/Mengano for men, Fulana/Mengana for women.

I guess our John/Jane Smith would be Pepe/Pepa García.