auxiliary language

Short definition from Wiktionary

Noun

1. 1
   Any language (whether constructed or natural), such as Volapük, Esperanto, Swahili, French, Russian or English, used or intended to be used (locally, regionally, nationally or internationally) for intercommunication by speakers of various other languages.
2. 2
   Ellipsis of international auxiliary language.
3. 3
   A local minority language which has official recognition.
4. 4
   A liturgical language, such as Latin, Sanskrit, or Old Church Slavonic, used in religious services.