diff --git a/data/web/inc/prerequisites.inc.php b/data/web/inc/prerequisites.inc.php
index 6cfdd1ca..b9737d64 100644
--- a/data/web/inc/prerequisites.inc.php
+++ b/data/web/inc/prerequisites.inc.php
@@ -194,7 +194,7 @@ require_once $_SERVER['DOCUMENT_ROOT'] . '/inc/sessions.inc.php';
 // Set language
 if (!isset($_SESSION['mailcow_locale']) && !isset($_COOKIE['mailcow_locale'])) {
   if ($DETECT_LANGUAGE && isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
-    $header_lang = strtolower(substr($_SERVER['HTTP_ACCEPT_LANGUAGE'], 0, 2));
+    $header_lang = strtolower($_SERVER['HTTP_ACCEPT_LANGUAGE']);
     if (array_key_exists($header_lang, $AVAILABLE_LANGUAGES)) {
       $_SESSION['mailcow_locale'] = $header_lang;
     }
diff --git a/data/web/inc/vars.inc.php b/data/web/inc/vars.inc.php
index 06783972..177c7dd9 100644
--- a/data/web/inc/vars.inc.php
+++ b/data/web/inc/vars.inc.php
@@ -101,8 +101,8 @@ $AVAILABLE_LANGUAGES = array(
   'sk' => 'Slovenčina (Slovak)',
   'sv' => 'Svenska (Swedish)',
   'uk' => 'Українська (Ukrainian)',
-  'zh_Hans' => '简体中文 (Simplified Chinese)',
-  'zh_Hant' => '繁體中文 (Traditional Chinese)',
+  'zh-cn' => '简体中文 (Simplified Chinese)',
+  'zh-tw' => '繁體中文 (Traditional Chinese)',
 );
 
 // Change theme (default: lumen)
diff --git a/data/web/lang/lang.zh_Hans.json b/data/web/lang/lang.zh-cn.json
similarity index 100%
rename from data/web/lang/lang.zh_Hans.json
rename to data/web/lang/lang.zh-cn.json
diff --git a/data/web/lang/lang.zh_Hant.json b/data/web/lang/lang.zh-tw.json
similarity index 100%
rename from data/web/lang/lang.zh_Hant.json
rename to data/web/lang/lang.zh-tw.json
diff --git a/data/web/templates/base.twig b/data/web/templates/base.twig
index 08376e71..f816d6cc 100644
--- a/data/web/templates/base.twig
+++ b/data/web/templates/base.twig
@@ -37,7 +37,7 @@
             {% for key, val in available_languages %}
             <li{% if mailcow_locale == key %} class="active"{% endif %}>
               <a href="?{{ query_string({'lang': key}) }}">
-                <span class="flag-icon flag-icon-{{ key }}"></span>{{ val }}
+                <span class="flag-icon flag-icon-{{ key[-2:] }}"></span>{{ val }}
               </a>
             </li>
             {% endfor %}
diff --git a/data/web/templates/index.twig b/data/web/templates/index.twig
index 056421fe..653b65c4 100644
--- a/data/web/templates/index.twig
+++ b/data/web/templates/index.twig
@@ -51,7 +51,7 @@
               {% for key, val in available_languages %}
                 <li{% if mailcow_locale == key %} class="active"{% endif %}>
                   <a href="?{{ query_string({'lang': key}) }}">
-                    <span class="flag-icon flag-icon-{{ key }}"></span>{{ val }}
+                    <span class="flag-icon flag-icon-{{ key[-2:] }}"></span>{{ val }}
                   </a>
                 </li>
               {% endfor %}