FLAG ICONSThe following flag icons generally refer to a certain language:

Abkhaz

Ainu

Albanian

Alsatian German

Amharic or other Ethiopian languages

Ancient Greek

Andalusian

Arabic

Arabic (alternative)

Arabic (alternative)

Aragonese

Armenian

Aromanian

Arpitan

Asturian

Auregnais

Azeri

Basque

Belarusian

Belarusian (alternative)

Bengali

Berber

Bosnian

Breton

Bulgarian

Burmese

Burushaski

Cajun French

Cantonese

Castilian

Catalan

Chamorro

Chechen

Chechen (alternative)

Cherokee

Cornish

Corsican

Crimean Tatar

Croatian

Czech

Danish

Dutch

East Frisian

Elfdalian

English

Estonian

Faroese

Flemish Dutch

Finnish

French

Frisian (West Frisian)

Friulan

Gaelic or Scots

Gagauz

Galician

Garifuna

Georgian

German

Gothic

Greek (modern or ancient)

Greenlandic

Guernésiais

Haitian Creole

Hawaiian

Hebrew

Hungarian

Icelandic

Indonesian or other languages of Indonesia

Ingush

Inuktitut

Irish

Italian

Japanese

Jamtlandic/Jämtska

Jèrriais

Karelian

Kashubian

Kazakh

Khakas

Khmer

Komi

Korean

Kurdish

Kyrghyz

Lao

Lakota

Lappish/Sámi

Latin

Latvian

Lezgian

Lingua Franca Nova

Lithuanian

Livonian

Luxembourgish German

Macedonian

Malagasy

Malay

Maltese

Manx

Maori

Meänkieli

Moldovan

Mongol

Montenegrin (Serbocroatian)

Normand

North Frisian

Norwegian (Bokmål + Nynorsk)

Navajo

Occitan

Old Norse

Ossetic

Palenquero

Papiamento

Persian

Polish

Portuguese

Québécois French

Quechua

Romani

Romanian

Romansh

Russian

Sakha

Sardinian

Sercquiais

Serbian

Sesotho

Simplified Chinese

Sinhala

Slovakian

Slovenian

Somali

Sorbian, Upper or Lower

Spanish

Swedish

Tagalog or languages of the Philippines

Tajik

Tatar

Thai

Tibetan

Traditional Chinese

Tswana

Turkish

Turkmen

Tuvan

Udmurt

Ukranian

Uyghur

Uzbek

Valencian (Catalan)

Vepsian

Vietnamese

Vietnamese (alternative)

Walloon

Welsh
These flag icons may refer to several languages:

African Union

Andean Community

Argentina

Aruba

Australia

Australian Aboriginal languages

Austria

Belgium

Bolivia

Brazil

Canada

CARICOM

Chile

Colombia

Congo, D.R.

Europe, European Union

Guinea

India

Iroquois

Jamaica

MERCOSUR/MERCOSUL

Mexico

Micronesia

Namibia

New Zealand

Nigeria

Tok Pisin or other Papuan languages

Pacific Islands Forum

Pakistan

Peru

Ryukyuan languages

Senegal

Sierra Leone

Solomon Islands

South Africa

South East Asia

Suriname

Switzerland

Tanzania, good for Swahili

UNASUR

United Kingdom

United States

Vanuatu
These flags refer to conlangs:

Any conlang

Esperanto

Ido

Klingon

Lingua Franca Nova

LotR languages

Na'vi

Quenya

Sindarin

Volapük
These icons refer to subdivisions of countries:
(note that other subdivisions may be present in the lists above)
Canada
Newfoundland and Labrador, Canada
Denmark
Bornholm
England
Greater London

Lancashire

Lincolnshire

Newcastle

Yorkshire
Finland
Lappi/Lappland

Åland (Ahvenanmaa), flag version

Österbotten/Pohjanmaa
Germany
Bayern (Bavaria)

Berlin

Brandenburg

Bremen

Hamburg

Mecklenburg-Vorpommern

Niedersachen (Lower Saxony)

Nordrhein-Westphalen (North Rhine-Westphalia)

Sachsen (Saxony)

Sachsen-Anhalt (Saxony-Anhalt)

Schleswig-Holstein
Japan
Osaka
Mexico
Sonora
Poland
Małopolska (Lesser Poland), Poland

Mazowsze (Masovia), Poland

Pomorze (Pomerania), Poland

Śląsk (Silesia), Poland

Wielkopolska (Greater Poland), Poland
Russia
Dagestan

Kabardino-Balkaria

Karelia
Sweden
Dalarna (Dalecarlia)

Skåne (Scania)

Skåne, flag version

Uppland

Västergötland

Östergötland
USA
Alabama

Arizona

California

Colorado

Louisiana/Louisiane

Maryland

Massachusetts

Mississippi

New Mexico

New York City

Ohio

Pennsylvania

Tennessee

Texas

Washington (state)
Flags generally use the ISO 639-3 code of the language associated with them.