Errata on FBref.com
How we determine player nationalities
1. Identify all pieces of evidence that may confer national affiliation to a player. This naturally includes appearances for senior or youth national teams, but also includes secondary pieces of evidence such as country of birth, family relationships, and stated preferences.
2. If a player has represented the senior team for a FIFA member association in an official competition, this will usually be their permanent and immutable primary nationality. If a player has represented a youth team (or the senior team in an unofficial competition) for a FIFA member association, this will be their permanent primary nationality unless and until they are granted approval by FIFA to change nationality. If a player has never represented a FIFA member association on any level, we are typically making an unscientific guess as to the player's nationality based on the other factors (country of birth, family, stated preferences).
3. Any other national affiliations we might identify that are different from the player's primary nationality is displayed as a secondary nationality. For players who have not represented the senior team for a FIFA member association in an official competition (or been approved for a one-time nationality change), any secondary nationalities a player might have may plausibly become the player's primary nationality in the future. Otherwise, secondary nationalities have no special meaning.