You don’t even need to go through world exploring and find read context clues from zandik notes to know he’s sumeran. He literally says it himself
"dottore could be from snezhnaya/fontaine/mondstadt" zandik zandik zandik zandik zandik zandik zandik HIS REAL NAME IS ZANDIK zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik zandik ZANDIK