Circular country flags from circle-flags v2.8.3, MIT licensed, copyright (c) HatScripts.
Only the nationalities the current squads use are here, not the whole set. A player from a country with no file falls back to the SportMonks flag at runtime via the onerror on the <img> - the same pattern the club badges use - so a missing file shows the right flag rather than nothing, and this directory only needs topping up when someone wants the sharper version.
Filenames are ISO 3166-1 alpha-2, lower-cased, except the four home nations, which ISO has no codes for: gb-eng, gb-sct, gb-wls, gb-nir. See src/_utils/flags.js, which is the only place that decides which file a nationality gets.