diff --git a/core/SwipeViewController.swift b/core/SwipeViewController.swift
index d509c31..399b326 100755
--- a/core/SwipeViewController.swift
+++ b/core/SwipeViewController.swift
@@ -91,8 +91,7 @@ class SwipeViewController: UIViewController, UIScrollViewDelegate, SwipeDocument
         self.book = SwipeBook(bookInfo: document, url: url, delegate: self)
 
         if let languages = self.book.languages(),
-            let language = languages.first,
-            let langId = language["id"] as? String {
+           let langId = preferredLangId(in: languages) {
             self.book.langId = langId
         }
 
@@ -118,6 +117,29 @@ class SwipeViewController: UIViewController, UIScrollViewDelegate, SwipeDocument
             callback(prefetcher.progress, nil)
         }
     }
+    
+    private func preferredLangId(in languages: [[String : Any]]) -> String? {
+        // 1st, find a lang id matching system language.
+        // A lang id can be in "en-US", "en", "zh-Hans-CN" or "zh-Hans" formats (with/without a country code).
+        // "en-US" or "zh-Hans-CN" formats with a country code have higher priority to match than "en" or "zh-Hans" without a country code.
+        if let systemLanguage = NSLocale.preferredLanguages.first?.components(separatedBy: "-") {
+            for rangeEnd in systemLanguage.indices.reversed() {
+                let langId = systemLanguage[0...rangeEnd].joined(separator: "-")
+                if languages.contains(where: { $0["id"] as? String == langId }) {
+                    return langId
+                }
+            }
+        }
+        // 2nd, use the default lang id ("*") if no lang id matches, and the default id exists.
+        if languages.contains(where: { $0["id"] as? String == "*" }) {
+            return "*"
+        }
+        // At last, use the first lang id in the language list if still no lang id is specified.
+        if let langId = languages.first?["id"] as? String {
+            return langId
+        }
+        return nil
+    }
 
     // <SwipeDocumentViewer> method
     func setDelegate(_ delegate:SwipeDocumentViewerDelegate) {