diff --git a/src/book/book.rs b/src/book/book.rs
index af23e785..fa65fcb5 100644
--- a/src/book/book.rs
+++ b/src/book/book.rs
@@ -15,7 +15,7 @@ pub fn load_book<P: AsRef<Path>>(
     cfg: &Config,
     build_opts: &BuildOpts,
 ) -> Result<LoadedBook> {
-    if cfg.language.has_localized_dir_structure() {
+    if cfg.has_localized_dir_structure() {
         match build_opts.language_ident {
             // Build a single book's translation.
             Some(_) => Ok(LoadedBook::Single(load_single_book_translation(
diff --git a/src/book/init.rs b/src/book/init.rs
index 3f489cdd..26eb6d7d 100644
--- a/src/book/init.rs
+++ b/src/book/init.rs
@@ -14,18 +14,18 @@ pub struct BookBuilder {
     create_gitignore: bool,
     config: Config,
     copy_theme: bool,
-    language_ident: String
+    language_ident: String,
 }
 
 fn add_default_language(cfg: &mut Config, language_ident: String) {
     let language = Language {
         name: String::from("English"),
-        default: true,
         title: None,
         authors: None,
         description: None,
     };
-    cfg.language.0.insert(language_ident, language);
+    cfg.language.0.insert(language_ident.clone(), language);
+    cfg.book.language = Some(language_ident);
 }
 
 impl BookBuilder {
@@ -41,13 +41,16 @@ impl BookBuilder {
             create_gitignore: false,
             config: cfg,
             copy_theme: false,
-            language_ident: language_ident
+            language_ident: language_ident,
         }
     }
 
     /// Get the output source directory of the builder.
     pub fn source_dir(&self) -> PathBuf {
-        let src = self.config.get_localized_src_path(Some(&self.language_ident)).unwrap();
+        let src = self
+            .config
+            .get_localized_src_path(Some(&self.language_ident))
+            .unwrap();
         self.root.join(src)
     }
 
@@ -125,8 +128,8 @@ impl BookBuilder {
 
         File::create(book_toml)
             .with_context(|| "Couldn't create book.toml")?
-        .write_all(&cfg)
-        .with_context(|| "Unable to write config to book.toml")?;
+            .write_all(&cfg)
+            .with_context(|| "Unable to write config to book.toml")?;
         Ok(())
     }
 
diff --git a/src/cmd/build.rs b/src/cmd/build.rs
index 29ddacc9..25116cb9 100644
--- a/src/cmd/build.rs
+++ b/src/cmd/build.rs
@@ -19,7 +19,7 @@ pub fn make_subcommand<'a, 'b>() -> App<'a, 'b> {
         .arg_from_usage("-o, --open 'Opens the compiled book in a web browser'")
         .arg_from_usage(
             "-l, --language=[language] 'Language to render the compiled book in.{n}\
-                         Only valid if the [languages] table in the config is not empty.{n}\
+                         Only valid if the [language] table in the config is not empty.{n}\
                          If omitted, builds all translations and provides a menu in the generated output for switching between them.'",
         )
 }
diff --git a/src/cmd/clean.rs b/src/cmd/clean.rs
index 98fdb505..4884146b 100644
--- a/src/cmd/clean.rs
+++ b/src/cmd/clean.rs
@@ -20,7 +20,7 @@ pub fn make_subcommand<'a, 'b>() -> App<'a, 'b> {
         )
         .arg_from_usage(
             "-l, --language=[language] 'Language to render the compiled book in.{n}\
-                         Only valid if the [languages] table in the config is not empty.{n}\
+                         Only valid if the [language] table in the config is not empty.{n}\
                          If omitted, builds all translations and provides a menu in the generated output for switching between them.'",
         )
 }
diff --git a/src/cmd/serve.rs b/src/cmd/serve.rs
index d768cadb..58f18acc 100644
--- a/src/cmd/serve.rs
+++ b/src/cmd/serve.rs
@@ -52,7 +52,7 @@ pub fn make_subcommand<'a, 'b>() -> App<'a, 'b> {
         .arg_from_usage("-o, --open 'Opens the book server in a web browser'")
         .arg_from_usage(
             "-l, --language=[language] 'Language to render the compiled book in.{n}\
-                         Only valid if the [languages] table in the config is not empty.{n}\
+                         Only valid if the [language] table in the config is not empty.{n}\
                          If omitted, builds all translations and provides a menu in the generated output for switching between them.'",
         )
 }
@@ -86,7 +86,7 @@ pub fn execute(args: &ArgMatches) -> Result<()> {
     let language: Option<String> = match build_opts.language_ident {
         // index.html will be at the root directory.
         Some(_) => None,
-        None => match book.config.language.default_language() {
+        None => match book.config.default_language() {
             // If book has translations, index.html will be under src/en/ or
             // similar.
             Some(lang_ident) => Some(lang_ident.clone()),
diff --git a/src/cmd/test.rs b/src/cmd/test.rs
index 0295547f..642b4d20 100644
--- a/src/cmd/test.rs
+++ b/src/cmd/test.rs
@@ -26,7 +26,7 @@ pub fn make_subcommand<'a, 'b>() -> App<'a, 'b> {
             .empty_values(false)
             .help("A comma-separated list of directories to add to {n}the crate search path when building tests"))
         .arg_from_usage("-l, --language=[language] 'Language to render the compiled book in.{n}\
-                         Only valid if the [languages] table in the config is not empty.{n}\
+                         Only valid if the [language] table in the config is not empty.{n}\
                          If omitted, builds all translations and provides a menu in the generated output for switching between them.'")
 }
 
diff --git a/src/cmd/watch.rs b/src/cmd/watch.rs
index d7841311..aa2f0c65 100644
--- a/src/cmd/watch.rs
+++ b/src/cmd/watch.rs
@@ -25,7 +25,7 @@ pub fn make_subcommand<'a, 'b>() -> App<'a, 'b> {
         .arg_from_usage("-o, --open 'Open the compiled book in a web browser'")
         .arg_from_usage(
             "-l, --language=[language] 'Language to render the compiled book in.{n}\
-                         Only valid if the [languages] table in the config is not empty.{n}\
+                         Only valid if the [language] table in the config is not empty.{n}\
                          If omitted, builds all translations and provides a menu in the generated output for switching between them.'",
         )
 }
diff --git a/src/config.rs b/src/config.rs
index 2341cc06..d1a4f68c 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -255,7 +255,7 @@ impl Config {
 
     /// Gets the language configured for a book.
     pub fn get_language<I: AsRef<str>>(&self, index: Option<I>) -> Result<Option<String>> {
-        match self.language.default_language() {
+        match self.default_language() {
             // Languages have been specified, assume directory structure with
             // language subfolders.
             Some(ref default) => match index {
@@ -342,7 +342,7 @@ impl Config {
     /// missing in a localization, any links to them will gracefully degrade to
     /// the files that exist in this directory.
     pub fn get_fallback_src_path(&self) -> PathBuf {
-        match self.language.default_language() {
+        match self.default_language() {
             // Languages have been specified, assume directory structure with
             // language subfolders.
             Some(default) => {
@@ -358,6 +358,31 @@ impl Config {
         }
     }
 
+    /// If true, mdBook should assume there are subdirectories under src/
+    /// corresponding to the localizations in the config. If false, src/ is a
+    /// single directory containing the summary file and the rest.
+    pub fn has_localized_dir_structure(&self) -> bool {
+        !self.language.0.is_empty()
+    }
+
+    /// Obtains the default language for this config.
+    pub fn default_language(&self) -> Option<String> {
+        if self.has_localized_dir_structure() {
+            let language_ident = self
+                .book
+                .language
+                .clone()
+                .expect("Config has [language] table, but `book.language` not was declared");
+            self.language.0.get(&language_ident).expect(&format!(
+                "Expected [language.{}] to be declared in book.toml",
+                language_ident
+            ));
+            Some(language_ident)
+        } else {
+            None
+        }
+    }
+
     fn from_legacy(mut table: Value) -> Config {
         let mut cfg = Config::default();
 
@@ -455,12 +480,28 @@ impl<'de> Deserialize<'de> for Config {
 
         if !language.0.is_empty() {
             let default_languages = language.0.iter().filter(|(_, lang)| lang.default).count();
-
-            if default_languages != 1  {
+            if default_languages != 1 {
                 return Err(D::Error::custom(
-                    "If [languages] table is specified, exactly one entry must be set as 'default'",
+                    "If the [language] table is specified, then `book.language` must be declared",
                 ));
             }
+            let language_ident = book.language.clone().unwrap();
+            if language.0.get(&language_ident).is_none() {
+                use serde::de::Error;
+                return Err(D::Error::custom(format!(
+                    "Expected [language.{}] to be declared in book.toml",
+                    language_ident
+                )));
+            }
+            for (ident, language) in language.0.iter() {
+                if language.name.is_empty() {
+                    use serde::de::Error;
+                    return Err(D::Error::custom(format!(
+                        "`name` property for [language.{}] must be non-empty",
+                        ident
+                    )));
+                }
+            }
         }
 
         Ok(Config {
@@ -492,7 +533,8 @@ impl Serialize for Config {
         }
 
         if !self.language.0.is_empty() {
-            let language_config = Value::try_from(&self.language).expect("should always be serializable");
+            let language_config =
+                Value::try_from(&self.language).expect("should always be serializable");
             table.insert("language", language_config);
         }
 
@@ -543,9 +585,7 @@ 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? (Deprecated.)
-    pub multilingual: bool,
-    /// The main language of the book. (Deprecated.)
+    /// The main language of the book.
     pub language: Option<String>,
 }
 
@@ -556,7 +596,6 @@ impl Default for BookConfig {
             authors: Vec::new(),
             description: None,
             src: PathBuf::from("src"),
-            multilingual: true,
             language: Some(String::from("en")),
         }
     }
@@ -830,9 +869,6 @@ pub struct LanguageConfig(pub HashMap<String, Language>);
 pub struct Language {
     /// Human-readable name of the language.
     pub name: String,
-    /// If true, this language is the default. There must be exactly one default
-    /// language in the config.
-    pub default: bool,
     /// Localized title of the book.
     pub title: Option<String>,
     /// The authors of the translation.
@@ -841,22 +877,7 @@ pub struct Language {
     pub description: Option<String>,
 }
 
-impl LanguageConfig {
-    /// If true, mdBook should assume there are subdirectories under src/
-    /// corresponding to the localizations in the config. If false, src/ is a
-    /// single directory containing the summary file and the rest.
-    pub fn has_localized_dir_structure(&self) -> bool {
-        self.default_language().is_some()
-    }
-
-    /// Returns the default language specified in the config.
-    pub fn default_language(&self) -> Option<&String> {
-        self.0
-            .iter()
-            .find(|(_, lang)| lang.default)
-            .map(|(lang_ident, _)| lang_ident)
-    }
-}
+impl LanguageConfig {}
 
 /// Allows you to "update" any arbitrary field in a struct by round-tripping via
 /// a `toml::Value`.
@@ -923,7 +944,6 @@ mod tests {
 
         [language.en]
         name = "English"
-        default = true
 
         [language.ja]
         name = "日本語"
@@ -940,7 +960,6 @@ mod tests {
             title: Some(String::from("Some Book")),
             authors: vec![String::from("Michael-F-Bryan <michaelfbryan@gmail.com>")],
             description: Some(String::from("A completely useless book")),
-            multilingual: true,
             src: PathBuf::from("source"),
             language: Some(String::from("ja")),
         };
@@ -981,7 +1000,6 @@ mod tests {
             String::from("en"),
             Language {
                 name: String::from("English"),
-                default: true,
                 title: None,
                 description: None,
                 authors: None,
@@ -991,7 +1009,6 @@ mod tests {
             String::from("ja"),
             Language {
                 name: String::from("日本語"),
-                default: false,
                 title: Some(String::from("なんかの本")),
                 description: Some(String::from("何の役にも立たない本")),
                 authors: Some(vec![String::from("Ruin0x11")]),
@@ -1354,10 +1371,10 @@ mod tests {
 
     #[test]
     #[should_panic(expected = "Invalid configuration file")]
-    fn validate_default_language() {
+    fn validate_config_default_language_must_exist_in_languages_table() {
         let src = r#"
-        [language.en]
-        name = "English"
+        [language.ja]
+        name = "日本語"
         "#;
 
         Config::from_str(src).unwrap();
@@ -1365,15 +1382,12 @@ mod tests {
 
     #[test]
     #[should_panic(expected = "Invalid configuration file")]
-    fn validate_default_language_2() {
+    fn validate_language_config_must_have_name() {
         let src = r#"
-        [language.en]
-        name = "English"
-        default = true
+        [book]
+        language = "en"
 
-        [language.fr]
-        name = "Français"
-        default = true
+        [language.en]
         "#;
 
         Config::from_str(src).unwrap();
diff --git a/tests/init.rs b/tests/init.rs
index 454f3bf0..c1b3f9ea 100644
--- a/tests/init.rs
+++ b/tests/init.rs
@@ -10,7 +10,13 @@ use tempfile::Builder as TempFileBuilder;
 /// are created.
 #[test]
 fn base_mdbook_init_should_create_default_content() {
-    let created_files = vec!["book", "src", "src/en", "src/en/SUMMARY.md", "src/en/chapter_1.md"];
+    let created_files = vec![
+        "book",
+        "src",
+        "src/en",
+        "src/en/SUMMARY.md",
+        "src/en/chapter_1.md",
+    ];
 
     let temp = TempFileBuilder::new().prefix("mdbook").tempdir().unwrap();
     for file in &created_files {
@@ -28,7 +34,7 @@ fn base_mdbook_init_should_create_default_content() {
     let contents = fs::read_to_string(temp.path().join("book.toml")).unwrap();
     assert_eq!(
         contents,
-        "[book]\nauthors = []\nlanguage = \"en\"\nmultilingual = true\nsrc = \"src\"\n[language.en]\ndefault = true\nname = \"English\"\n"
+        "[book]\nauthors = []\nlanguage = \"en\"\nsrc = \"src\"\n[language.en]\nname = \"English\"\n"
     );
 }
 
@@ -66,7 +72,13 @@ fn run_mdbook_init_should_create_content_from_summary() {
 /// files, then call `mdbook init`.
 #[test]
 fn run_mdbook_init_with_custom_book_and_src_locations() {
-    let created_files = vec!["out", "in", "in/en", "in/en/SUMMARY.md", "in/en/chapter_1.md"];
+    let created_files = vec![
+        "out",
+        "in",
+        "in/en",
+        "in/en/SUMMARY.md",
+        "in/en/chapter_1.md",
+    ];
 
     let temp = TempFileBuilder::new().prefix("mdbook").tempdir().unwrap();
     for file in &created_files {