Galach is the universal lingua franca in the fictional Dune universe created by Frank Herbert, described in the "Terminology of the Imperium" appendix of Dune as "Hybrid Inglo-Slavic with strong traces of cultural-specialization terms adopted during the long chain of human migrations," indicating that the language is either a creole or mixed language likely descended from English and Russian. As the official language of the Imperium, Galach is used in official texts and in the Royal Court on Kaitain. Other languages are used at a local level and many people will only use Galach when they meet someone from another planet.
It also appears as the universal language in other novels by Herbert, such as Whipping Star and The Dosadi Experiment.
Willis E. McNelly's The Dune Encyclopedia has an article on Galach that consists entirely of summaries of phonological, morphological, and syntactic changes from "Proto-Galach" (modern English) through various stages of development to "Atreidean Galach," the state of the language at the time of Herbert's novels. The names of various contributors to the Encyclopedia had the phonological alterations applied to their names for use in the fictional context; for example, "Gwilit Mignail" appears as a name in the text, the Galach version of "Willis McNelly".