The multilingual option was removed in 0.5, and never had any effect

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#05-migration-guide

book.toml

@@ -4,7 +4,6 @@
 title = "Synapse"
 authors = ["The Matrix.org Foundation C.I.C."]
 language = "en"
-multilingual = false
 
 # The directory that documentation files are stored in
 src = "docs"