We looked around for this one but were not able to find a default string for country select that has country codes as values so we decided to share one. This is particularly important if you want to use Salesforce to generate country based reports.
Select::,
Afghanistan::AF,
Albania::AL,
Algeria::DZ,
American Samoa::AS,
Andorra::AD,
Angola::AG,
Anguilla::AI,
Antigua & Barbuda::AG,
Argentina::AR,
Armenia::AA,
Aruba::AW,
Australia::AU,
Austria::AT,
Azerbaijan::AZ,
Bahamas::BS,
Bahrain::BH,
Bangladesh::BD,
Barbados::BB,
Belarus::BY,
Belgium::BE,
Belize::BZ,
Benin::BJ,
Bermuda::BM,
Bhutan::BT,
Bolivia::BO,
Bonaire::BL,
Bosnia & Herzegovina::BA,
Botswana::BW,
Brazil::BR,
British Indian Ocean Ter::BC,
Brunei::BN,
Bulgaria::BG,
Burkina Faso::BF,
Burundi::BI,
Cambodia::KH,
Cameroon::CM,
Canada::CA,
Canary Islands::IC,
Cape Verde::CV,
Cayman Islands::KY,
Central African Republic::CF,
Chad::TD,
Channel Islands::CD,
Chile::CL,
China::CN,
Christmas Island::CI,
Cocos Island::CS,
Colombia::CO,
Comoros::CC,
Congo::CG,
Cook Islands::CK,
Costa Rica::CR,
Cote D\'Ivoire::CT,
Croatia::HR,
Cuba::CU,
Curacao::CB,
Cyprus::CY,
Czech Republic::CZ,
Denmark::DK,
Djibouti::DJ,
Dominica::DM,
Dominican Republic::DO,
East Timor::TM,
Ecuador::EC,
Egypt::EG,
El Salvador::SV,
Equatorial Guinea::GQ,
Eritrea::ER,
Estonia::EE,
Ethiopia::ET,
Falkland Islands::FA,
Faroe Islands::FO,
Fiji::FJ,
Finland::FI,
France::FR,
French Guiana::GF,
French Polynesia::PF,
French Southern Ter::FS,
Gabon::GA,
Gambia::GM,
Georgia::GE,
Germany::DE,
Ghana::GH,
Gibraltar::GI,
Great Britain::GB,
Greece::GR,
Greenland::GL,
Grenada::GD,
Guadeloupe::GP,
Guam::GU,
Guatemala::GT,
Guinea::GN,
Guyana::GY,
Haiti::HT,
Hawaii::HW,
Honduras::HN,
Hong Kong::HK,
Hungary::HU,
Iceland::IS,
India::IN,
Indonesia::ID,
Iran::IA,
Iraq::IQ,
Ireland::IR,
Isle of Man::IM,
Israel::IL,
Italy::IT,
Jamaica::JM,
Japan::JP,
Jordan::JO,
Kazakhstan::KZ,
Kenya::KE,
Kiribati::KI,
Korea North::NK,
Korea South::KS,
Kuwait::KW,
Kyrgyzstan::KG,
Laos::LA,
Latvia::LV,
Lebanon::LB,
Lesotho::LS,
Liberia::LR,
Libya::LY,
Liechtenstein::LI,
Lithuania::LT,
Luxembourg::LU,
Macau::MO,
Macedonia::MK,
Madagascar::MG,
Malaysia::MY,
Malawi::MW,
Maldives::MV,
Mali::ML,
Malta::MT,
Marshall Islands::MH,
Martinique::MQ,
Mauritania::MR,
Mauritius::MU,
Mayotte::ME,
Mexico::MX,
Midway Islands::MI,
Moldova::MD,
Monaco::MC,
Mongolia::MN,
Montserrat::MS,
Morocco::MA,
Mozambique::MZ,
Myanmar::MM,
Nambia::NA,
Nauru::NU,
Nepal::NP,
Netherland Antilles::AN,
Netherlands (Holland Europe)::NL,
Nevis::NV,
New Caledonia::NC,
New Zealand::NZ,
Nicaragua::NI,
Niger::NE,
Nigeria::NG,
Niue::NW,
Norfolk Island::NF,
Norway::NO,
Oman::OM,
Pakistan::PK,
Palau Island::PW,
Palestine::PS,
Panama::PA,
Papua New Guinea::PG,
Paraguay::PY,
Peru::PE,
Philippines::PH,
Pitcairn Island::PO,
Poland::PL,
Portugal::PT,
Puerto Rico::PR,
Qatar::QA,
Republic of Montenegro::ME,
Republic of Serbia::RS,
Reunion::RE,
Romania::RO,
Russia::RU,
Rwanda::RW,
St Barthelemy::NT,
St Eustatius::EU,
St Helena::HE,
St Kitts-Nevis::KN,
St Lucia::LC,
St Maarten::MB,
St Pierre & Miquelon::PM,
St Vincent & Grenadines::VC,
Saipan::SP,
Samoa::SO,
Samoa American::AS,
San Marino::SM,
Sao Tome & Principe::ST,
Saudi Arabia::SA,
Senegal::SN,
Seychelles::SC,
Sierra Leone::SL,
Singapore::SG,
Slovakia::SK,
Slovenia::SI,
Solomon Islands::SB,
Somalia::OI,
South Africa::ZA,
Spain::ES,
Sri Lanka::LK,
Sudan::SD,
Suriname::SR,
Swaziland::SZ,
Sweden::SE,
Switzerland::CH,
Syria::SY,
Tahiti::TA,
Taiwan::TW,
Tajikistan::TJ,
Tanzania::TZ,
Thailand::TH,
Togo::TG,
Tokelau::TK,
Tonga::TO,
Trinidad & Tobago::TT,
Tunisia::TN,
Turkey::TR,
Turkmenistan::TU,
Turks & Caicos Is::TC,
Tuvalu::TV,
Uganda::UG,
Ukraine::UA,
United Arab Emirates::AE,
United Kingdom::GB,
United States of America::US,
Uruguay::UY,
Uzbekistan::UZ,
Vanuatu::VU,
Vatican City State::VS,
Venezuela::VE,
Vietnam::VN,
Virgin Islands (Brit)::VB,
Virgin Islands (USA)::VA,
Wake Island::WK,
Wallis & Futana Is::WF,
Yemen::YE,
Zaire::ZR,
Zambia::ZM,
Zimbabwe::ZW
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Hope this helps.