This should work:
collection.EnsureIndex(IndexKeys.Text("a", "b").Ascending("c"), IndexOptions.SetTextLanguageOverride("idioma").SetName("custom").SetTextDefaultLanguage("spanish"));
https://jira.mongodb.org/browse/CSHARP-874
https://github.com/mongodb/mongo-csharp-driver/commit/1e7db3bedb3bee1b0ccecdb5f8ff39854526213a