Revert: Remove the book.multilingual field and don't serialize it

examples/nop-preprocessor.rs

@@ -117,6 +117,7 @@ mod nop_lib {
                         "book": {
                             "authors": ["AUTHOR"],
                             "language": "en",
+                            "multilingual": false,
                             "src": "src",
                             "title": "TITLE"
                         },

src/config.rs

@@ -420,6 +420,11 @@ pub struct BookConfig {
     pub description: Option<String>,
     /// Location of the book source relative to the book's root directory.
     pub src: PathBuf,
+    /// Does this book support more than one language?
+    // TODO: Remove this field in 0.5, it is unused:
+    // https://github.com/rust-lang/mdBook/issues/2636
+    #[serde(skip_serializing)]
+    pub multilingual: bool,
     /// The main language of the book.
     pub language: Option<String>,
     /// The direction of text in the book: Left-to-right (LTR) or Right-to-left (RTL).
@@ -434,6 +439,7 @@ impl Default for BookConfig {
             authors: Vec::new(),
             description: None,
             src: PathBuf::from("src"),
+            multilingual: false,
             language: Some(String::from("en")),
             text_direction: None,
         }
@@ -817,6 +823,7 @@ mod tests {
         title = "Some Book"
         authors = ["Michael-F-Bryan <[email protected]>"]
         description = "A completely useless book"
+        multilingual = true
         src = "source"
         language = "ja"
 
@@ -855,6 +862,7 @@ mod tests {
             title: Some(String::from("Some Book")),
             authors: vec![String::from("Michael-F-Bryan <[email protected]>")],
             description: Some(String::from("A completely useless book")),
+            multilingual: true,
             src: PathBuf::from("source"),
             language: Some(String::from("ja")),
             text_direction: None,