Probably most familiar are the country IDs from the license plates, that's
why I chose them:
AUS - Australia
A - Austria
B - Belgium
BB - Barbados
BG - Bulgaria
BM - Bermuda
BR - Brazil
CDN - Canada
CH - Switzerland
CZ - Czech Republic
DK - Denmark
D - Germany
E - Spain
EST - Estonia
FIN - Finland
F - France
GB - Great Britain
GBG - Guernsey
GBZ - Gibraltar
GR - Greece
H - Hungary
HK - Hong Kong
HR - Croatia
I - Italy
IL - Israel
IRL - Ireland
IS - Iceland
J - Japan
JE - Jersey
LB - Lebanon
LT - Lithuania
LV - Latvia
MT - Malta
N - Norway
NL - Netherlands
NZ - New Zealand
P - Portugal
PE - Peru
PL - Poland
RA - Argentine
RO - Romania
ROK - South Korea
RUS - Russia
S - Sweden
SA - Saudi Arabia
SK - Slovakia
TR - Turkey
USA - United States of America
VE - Venezuela
YU - former Yugoslavia
ZA - South Africa
ZW - Zimbabwe
If you need a country code not in the list above, please let me know!