diff --git a/Web/scripts/admin/schedule.js b/Web/scripts/admin/schedule.js
index ecbbbcb2ed..0d21f99d8e 100644
--- a/Web/scripts/admin/schedule.js
+++ b/Web/scripts/admin/schedule.js
@@ -666,7 +666,7 @@ function ScheduleManagement(opts) {
         allDaySlot: false,
         initialDate: opts.calendarOptions.defaultDate,
         initialView: 'dayGridMonth',
-        locale: (document.documentElement.lang || 'en').replace('_', '-'),
+        locale: document.documentElement.lang.slice(0, 2) || 'en',
         views: {
           dayGridMonth: { buttonText: opts.calendarOptions.buttonText.month },
           timeGridWeek: { buttonText: opts.calendarOptions.buttonText.week },
diff --git a/Web/scripts/calendar.js b/Web/scripts/calendar.js
index f4df0717f8..64a8484ee1 100644
--- a/Web/scripts/calendar.js
+++ b/Web/scripts/calendar.js
@@ -16,9 +16,6 @@ function Calendar(opts) {
   };
 
   // Helper functions for FullCalendar v6
-  function normalizeLocale(locale) {
-    return (locale || document.documentElement.lang || 'en').replace('_', '-');
-  }
 
   function mapCurrentViewToLegacy(viewType) {
     switch (viewType) {
@@ -93,7 +90,7 @@ function Calendar(opts) {
       weekNumbers: _options.showWeekNumbers,
       initialView: mapLegacyViewToCurrent(_options.view),
       initialDate: _options.defaultDate,
-      locale: normalizeLocale(_options.locale),
+      locale: document.documentElement.lang.slice(0, 2) || 'en',
       events: {
         url: _options.eventsUrl,
         extraParams: function () {
diff --git a/lang/ar.php b/lang/ar.php
index 94ee5f3c65..97a1063b30 100644
--- a/lang/ar.php
+++ b/lang/ar.php
@@ -1103,6 +1103,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'en';
+        return 'ar';
     }
 }
diff --git a/lang/cz.php b/lang/cz.php
index 9db541ce23..c46bbee401 100644
--- a/lang/cz.php
+++ b/lang/cz.php
@@ -911,6 +911,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'cz';
+        return 'cs';
     }
 }
diff --git a/lang/du_be.php b/lang/du_be.php
index 02c04fe0cd..9c45b6a976 100644
--- a/lang/du_be.php
+++ b/lang/du_be.php
@@ -481,6 +481,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'en_us';
+        return 'nl-BE';
     }
 }
diff --git a/lang/du_nl.php b/lang/du_nl.php
index c8b102cbb5..36df2abab9 100644
--- a/lang/du_nl.php
+++ b/lang/du_nl.php
@@ -1084,6 +1084,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'du_nl';
+        return 'nl-NL';
     }
 }
diff --git a/lang/ee_ee.php b/lang/ee_ee.php
index c0d2031573..2df0cc2c94 100644
--- a/lang/ee_ee.php
+++ b/lang/ee_ee.php
@@ -186,7 +186,6 @@ protected function _LoadStrings()
         $strings['ReservationDescription'] = 'Broneeringu kirjeldus';
         $strings['ResourceList'] = 'Broneeritavad väljakud';
         $strings['Accessories'] = 'Lisavarustus';
-        ;
         $strings['InvitationList'] = 'Kutsutud inimesed';
         $strings['AccessoryName'] = 'Lisavarustuse nimi';
         $strings['QuantityAvailable'] = 'Kogus saadaval';
@@ -862,6 +861,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'ee';
+        return 'et';
     }
 }
diff --git a/lang/no_no.php b/lang/no_no.php
index 4369d0011e..fbba7f4724 100644
--- a/lang/no_no.php
+++ b/lang/no_no.php
@@ -728,6 +728,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'no_no';
+        return 'nb-NO';
     }
 }
diff --git a/lang/si_si.php b/lang/si_si.php
index f3ae36e2c6..d3ff6197e0 100644
--- a/lang/si_si.php
+++ b/lang/si_si.php
@@ -3,7 +3,7 @@
 /**
 Modified by Alenka Kavčič (alenka.kavcic@fri.uni-lj.si), UL FRI, July 2015
 Translated and adapted for Slovenian language
-*/
+ */
 
 require_once('Language.php');
 require_once('en_gb.php');
@@ -749,6 +749,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'si-si';
+        return 'sl-SI';
     }
 }
diff --git a/lang/tr_tr.php b/lang/tr_tr.php
index 00cfe9d836..3efc190332 100644
--- a/lang/tr_tr.php
+++ b/lang/tr_tr.php
@@ -881,6 +881,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'en';
+        return 'tr';
     }
 }
diff --git a/lang/vn_vn.php b/lang/vn_vn.php
index 0d9826ccc6..a75e8d0d89 100644
--- a/lang/vn_vn.php
+++ b/lang/vn_vn.php
@@ -839,6 +839,6 @@ protected function _LoadLetters()
 
     protected function _GetHtmlLangCode()
     {
-        return 'en';
+        return 'vi';
     }
 }