noun
auxiliary language
A language used for communication between groups who do not share a native language, such as Esperanto or a pidgin. Often used in linguistics and language planning contexts.
エスペラントは最も有名な補助言語の一つだ。
Esperanto is one of the most famous auxiliary languages.
国際会議では英語が事実上の補助言語として使われている。
English is used as a de facto auxiliary language at international conferences.
Compound of 補助 (hojo, 'assistance, auxiliary') and 言語 (gengo, 'language'). A straightforward modern technical term.