As a really rough rule of thumb, in both Japanese and South Korean cultures, it would be fairly improper to address or refer to a "stranger" or any non-close-fr

As a really rough rule of thumb, in both Japanese and South Korean cultures, it would be fairly improper to address or refer to a "stranger" or any non-close-friend [1] by their given name. A jersey nameplate would absolutely constitute a stranger/formal context, so family name would be expected here. Even if you did not know that pretty universally, jersey nameplates/tags (and nameplates writ large) with only one name will be the family name. 1 - "stranger/non-close-friend" here means someone who you aren't already a) quite familiar with and b) in the age bracket / position in hierarchy c) in an informal friendly context. Any teacher, boss, employee, acquaintance, professional, etc, it'll be family name. Classmates, close friends, family members will usually use given name (in informal context - code switching). As an amusing aside, this is a bit of an in-joke in the anime Mushoku Tensei (Jobless Reincarnation), where the main character is often using surnames and the formal or even Keigo register with his rough-and-tumble party mates and they are like "yo why are you so formal?". But also, Park and Moon are common Korean family names. Because like the near-majority of South Koreans have the Kim surname, you'll often see initials like "H S KIM"