An international auxiliary language (sometimes abbreviated as IAL or auxlang) is a language used (or to be used in the future) for communication between people from different nations who do not share a common native language. Although proposals have been made for existing languages to serve as an official international auxiliary language, the concept has been most commonly associated with constructed languages such as Esperanto which were designed from the beginning to serve this purpose. Proponents of Esperanto often use the term planned language instead, derived from the Esperanto word planlingvo (but this is somewhat ambiguous since it could also refer to constructed languages in general). See also lingua franca.

The term "auxiliary" implies that it is intended to be an additional language for the people of the world, rather than replace their native languages. Often, the phrase is used to refer to constructed languages proposed specifically to ease worldwide international communication, such as Esperanto, Ido, Interlingua, Volapük, Glosa, Folkspraak, Mondlango, Lingua Franca Nova and others. However, it can also refer to the concept of such a language being determined by international consensus, including even a natural language so chosen.

Invented auxiliary languages are not widely used; nor have natural languages such as English and French penetrated universally, as some people imagine. Moreover, advocates of various languages disagree about which language should be universal. To overcome these difficulties, it has been proposed that some language (natural or invented) be chosen by consensus of officials elected by the nations of the world, perhaps through the United Nations, in consultation with experts of various disciplines, a top-down approach. There would be a spoken and a written form; the adoption of an official script for the blind has also been proposed, to correspond to the chosen written international language. The language would be implemented in each nation as an additional (second) language, alongside the national languages. A bottom-up strategy tries to spread the language among ordinary users, so that it becomes the de facto standard.

However, the idea has not yet spread as widely as intended. Some people see the need for an official political endorsement from the nations of the world, backed by resources for instruction and implementation.

An international auxiliary sign language has been developed by deaf people who meet regularly at international forums such as sporting events or in political organisations. Previously referred to as Gestuno but now more commonly simply as 'international sign', the language has continued to develop since the first signs were standardised in 1973, and it is now in widespread use. International sign is distinct in many ways from spoken IALs; many signs are iconic and signers tend to insert these signs into the grammar of their own sign language, with an emphasis on visually intuitive gestures and mime.

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