diff --git a/app/assets/javascripts/app/lib/app_post/i18n.coffee b/app/assets/javascripts/app/lib/app_post/i18n.coffee
index c8bc27895..3d5e50476 100644
--- a/app/assets/javascripts/app/lib/app_post/i18n.coffee
+++ b/app/assets/javascripts/app/lib/app_post/i18n.coffee
@@ -86,22 +86,33 @@ class App.i18n
     _instance.mapTime
 
   @detectBrowserLocale: ->
-    return 'en-us' if !window.navigator.userLanguage && !window.navigator.language
-
     if window.navigator.languages
-      allLocales = App.Locale.all()
       for browserLocale in window.navigator.languages
-        for localAllLocale in allLocales
-          if browserLocale is localAllLocale.locale
-            return localAllLocale.locale
+        if local = @findLocalLocale(browserLocale)
+          return local
 
-      for browserLocale in window.navigator.languages
-        browserLocale = browserLocale.substr(0, 2)
-        for localAllLocale in allLocales
-          if browserLocale is localAllLocale.alias
-            return localAllLocale.locale
+    if window.navigator.language
+      if local = @findLocalLocale(window.navigator.language)
+        return local
 
-    window.navigator.userLanguage || window.navigator.language || 'en-us'
+    if window.navigator.userLanguage
+      if local = @findLocalLocale(window.navigator.userLanguage)
+        return local
+
+    return 'en-us'
+
+  @findLocalLocale: (given) ->
+    givenLower = given.toLowerCase()
+
+    for local in App.Locale.all()
+      if givenLower == local.locale.toLowerCase()
+        return local.locale.toLowerCase()
+
+    givenAlias = given.substr(0, 2).toLowerCase()
+
+    for local in App.Locale.all()
+      if givenAlias == local.alias.toLowerCase()
+        return local.locale.toLowerCase()
 
   @detectBrowserTimezone: ->
     return if !window.Intl
diff --git a/app/views/tests/i18n.html.erb b/app/views/tests/i18n.html.erb
new file mode 100644
index 000000000..6ff3e0590
--- /dev/null
+++ b/app/views/tests/i18n.html.erb
@@ -0,0 +1,14 @@
+<link rel="stylesheet" href="/assets/tests/qunit-1.21.0.css">
+<script src="/assets/tests/qunit-1.21.0.js"></script>
+<script src="/assets/tests/i18n.js"></script>
+
+<style type="text/css">
+body {
+  padding-top: 0px;
+}
+</style>
+
+<script type="text/javascript">
+</script>
+
+<div id="qunit" class="u-dontfold"></div>
diff --git a/config/routes/test.rb b/config/routes/test.rb
index 0b41a07a1..dce9d16de 100644
--- a/config/routes/test.rb
+++ b/config/routes/test.rb
@@ -4,6 +4,7 @@ Zammad::Application.routes.draw do
   match '/tests_session',                     to: 'tests#session',                    via: :get
   match '/tests_local_storage',               to: 'tests#local_storage',              via: :get
   match '/tests_ui',                          to: 'tests#ui',                         via: :get
+  match '/tests_i18n',                        to: 'tests#i18n',                       via: :get
   match '/tests_model',                       to: 'tests#model',                      via: :get
   match '/tests_model_binding',               to: 'tests#model_binding',              via: :get
   match '/tests_model_ui',                    to: 'tests#model_ui',                   via: :get
diff --git a/public/assets/tests/i18n.js b/public/assets/tests/i18n.js
new file mode 100644
index 000000000..a4fdb2419
--- /dev/null
+++ b/public/assets/tests/i18n.js
@@ -0,0 +1,58 @@
+test('i18n .detectBrowserLocale', function() {
+  var original_userLanguage = window.navigator.userLanguage
+  var original_language     = window.navigator.language
+  var original_languages    = window.navigator.languages
+
+  var reset = function(){
+    window.navigator.userLanguage = original_userLanguage
+    window.navigator.language     = original_language
+    window.navigator.languages    = original_languages
+  }
+
+  var mock = function(languages, language, userLanguage){
+    Object.defineProperty(window.navigator, 'language', {value: language, configurable: true });
+    Object.defineProperty(window.navigator, 'languages', {value: languages, configurable: true });
+    Object.defineProperty(window.navigator, 'userLanguage', {value: userLanguage, configurable: true });
+  }
+
+  mock(undefined, undefined, undefined)
+  equal(App.i18n.detectBrowserLocale(), 'en-us')
+
+  mock(undefined, undefined, 'non-existant');
+  equal(App.i18n.detectBrowserLocale(), 'en-us')
+
+  mock(undefined, undefined, 'en');
+  equal(App.i18n.detectBrowserLocale(), 'en-us')
+
+  mock(undefined, undefined, 'lt');
+  equal(App.i18n.detectBrowserLocale(), 'lt')
+
+  mock(undefined, 'lt', 'lv');
+  equal(App.i18n.detectBrowserLocale(), 'lt')
+
+  mock(undefined, 'en', 'lv');
+  equal(App.i18n.detectBrowserLocale(), 'en-us')
+
+  mock(['en'], 'lt', 'lv');
+  equal(App.i18n.detectBrowserLocale(), 'en-us')
+
+  mock(['en-us'], 'lt', 'lv');
+  equal(App.i18n.detectBrowserLocale(), 'en-us')
+
+  mock(['en-US'], 'lt', 'lv');
+  equal(App.i18n.detectBrowserLocale(), 'en-us')
+
+  mock(['lt', 'en-US'], 'lt', 'lv');
+  equal(App.i18n.detectBrowserLocale(), 'lt')
+
+  mock(['en-GB', 'en-US'], 'lt', 'lv');
+  equal(App.i18n.detectBrowserLocale(), 'en-gb')
+
+  mock(['en-XYZ'], 'lt', 'lv');
+  equal(App.i18n.detectBrowserLocale(), 'en-us')
+
+  mock(['xyz', 'lt'], 'lv', undefined);
+  equal(App.i18n.detectBrowserLocale(), 'lt')
+
+  reset()
+})
diff --git a/spec/system/js/q_unit_spec.rb b/spec/system/js/q_unit_spec.rb
index 7048b0308..c30117f86 100644
--- a/spec/system/js/q_unit_spec.rb
+++ b/spec/system/js/q_unit_spec.rb
@@ -24,6 +24,10 @@ RSpec.describe 'QUnit', type: :system, authenticated: false, set_up: true, webso
     async_q_unit_tests('core')
   end
 
+  it 'I18n' do
+    async_q_unit_tests('i18n')
+  end
+
   context 'UI' do
 
     it 'Base' do