The use of flag icons, particularly national flags, for languages is a common practice. Such icons have long been used on tourist attraction signage, and elsewhere in the tourism space, but have found wider use in website localization where UX limitations have become apparent.^[1][2]

National flags are the most commonly used flag icons for representing languages. They are generally chosen because they either represent the language's origin (e.g. the flag of Spain used over the flag of Mexico) or the highest number of native speakers (e.g. the flag of the United States over the Flag of England).^[3]

• 
  The flag of Spain is often used to represent Spanish, despite having fewer native speakers than Mexico
• 
  The flag of United States is often used to represent English, despite not being its origin

A diagonally divided flag between two or more nation states may be used when more than one country is a major user of a language. Examples of this are the flags of the United Kingdom, the United States and Canada to indicate the English language, the flags of China and Taiwan to represent Mandarin, the flags of France, Belgium, and Canada to represent the French language, the flags of Spain and Mexico to represent the Spanish language, and the flags of Portugal and Brazil to represent the Portuguese language.^[4][5]

• 
  A mixed national flag of the US and the UK, representing English
• 
  A mixed national flag of France, Canada and Belgium, representing French
• 
  A mixed national flag of Spain and Mexico, representing Spanish
• 
  A mixed national flag of Portugal and Brazil, representing Portuguese

Some international linguistic communities have flags which encompass all the speakers of a language while avoiding the symbolism of national flags, though they are not as widely recognized.^[6] Some, like the flag of Esperanto, the Yiddish flag or the Arabic flag have been designed specifically as symbols for languages themselves rather than for organizations which link nations that share the same language.^[7]

• 
  The flag of the International Organisation of La Francophonie, representing French
• 
  The flag of the Community of Portuguese Speaking Countries, representing Portuguese
• 
  The Verda Stelo flag, representing Esperanto
• 
  The komets alef flag as used by Duolingo, representing Yiddish
• 
  The flag of the letter Ḍād, representing Arabic

Some international organizations do not link nations speficially through language, but nonetheless encompass all the regions where one language is spoken. The flags for such organizations, like the flag of the Arab League, are therefore sometimes used to represent those languages.^[8]

National flags can also be used to distinguish between different written standards for a single language. For example, the Flag of Taiwan is often used for Traditional Chinese and the Flag of the People's Republic of China for Simplified Chinese.^[9]

• 
  Flag of Taiwan, representing Traditional Chinese
• 
  Flag of China, representing Simplified Chinese

Some Euronet ATMs (automated teller machines) display the Irish flag as a symbol for the English language (usually UK flag or English flag). In the media, this was speculated to be a response to Brexit, with the Republic of Ireland as one of the only two Anglophone nations left in the European Union (another being Malta). Dr. Oetker have been observed doing the same.^[10] The Irish flag is more usually used to signify the Irish language.^[11]

The use of flag icons for languages has been criticized as poor design, with some going as far as to call them harmful.^{[12][13][better source needed]} The symbolism of a flag introduces politicization, and often ambiguity. The use of a national flag disregards the fact that many languages are natively spoken in several nation states, and many nations have several major languages.^[14] Alternatives include using the native names of languages or their language codes, possibly under a generic symbol of translation such as the Language Icon.^{[citation needed]}

• 
  Sign in Tenerife, Spain uses for the Spanish text, for the English text and for the German text.
• 
  Sign in San Francisco, Córdoba, Argentina; unusually, the Spanish language is indicated by an Argentine flag, while below it is Italian and Piedmontese text with the flags of Italy and Flag of Piedmont.
• 
  Sign in Guernsey using the Flag of Guernsey to symbolise Guernésiais.

Wikimedia Commons has media related to Flags indicating languages.

• Flags are not languages (blog)