diff --git a/content/base/src/nsGkAtomList.h b/content/base/src/nsGkAtomList.h
index c887a8545be..61c05906eb9 100644
--- a/content/base/src/nsGkAtomList.h
+++ b/content/base/src/nsGkAtomList.h
@@ -747,7 +747,6 @@ GK_ATOM(oninvalid, "oninvalid")
 GK_ATOM(onkeydown, "onkeydown")
 GK_ATOM(onkeypress, "onkeypress")
 GK_ATOM(onkeyup, "onkeyup")
-GK_ATOM(onlanguagechange, "onlanguagechange")
 GK_ATOM(onlevelchange, "onlevelchange")
 GK_ATOM(onLoad, "onLoad")
 GK_ATOM(onload, "onload")
diff --git a/dom/base/Navigator.cpp b/dom/base/Navigator.cpp
index e8dd0d08327..cd625615ef6 100644
--- a/dom/base/Navigator.cpp
+++ b/dom/base/Navigator.cpp
@@ -351,87 +351,59 @@ Navigator::GetAppName(nsAString& aAppName)
 }
 
 /**
- * Returns the value of Accept-Languages (HTTP header) as a nsTArray of
- * languages. The value is set in the preference by the user ("Content
- * Languages").
+ * JS property navigator.language, exposed to web content.
+ * Take first value from Accept-Languages (HTTP header), which is
+ * the "content language" freely set by the user in the Pref window.
  *
- * "en", "en-US" and "i-cherokee" and "" are valid languages tokens.
+ * Do not use UI language (chosen app locale) here.
+ * See RFC 2616, Section 15.1.4 "Privacy Issues Connected to Accept Headers"
  *
- * An empty array will be returned if there is no valid languages.
+ * "en", "en-US" and "i-cherokee" and "" are valid.
+ * Fallback in case of invalid pref should be "" (empty string), to
+ * let site do fallback, e.g. to site's local language.
  */
-void
-Navigator::GetAcceptLanguages(nsTArray<nsString>& aLanguages)
+NS_IMETHODIMP
+Navigator::GetLanguage(nsAString& aLanguage)
 {
   // E.g. "de-de, en-us,en".
   const nsAdoptingString& acceptLang =
     Preferences::GetLocalizedString("intl.accept_languages");
 
-  // Split values on commas.
+  // Take everything before the first "," or ";", without trailing space.
   nsCharSeparatedTokenizer langTokenizer(acceptLang, ',');
-  while (langTokenizer.hasMoreTokens()) {
-    nsDependentSubstring lang = langTokenizer.nextToken();
+  const nsSubstring &firstLangPart = langTokenizer.nextToken();
+  nsCharSeparatedTokenizer qTokenizer(firstLangPart, ';');
+  aLanguage.Assign(qTokenizer.nextToken());
 
-    // Replace "_" with "-" to avoid POSIX/Windows "en_US" notation.
-    // NOTE: we should probably rely on the pref being set correctly.
-    if (lang.Length() > 2 && lang[2] == char16_t('_')) {
-      lang.Replace(2, 1, char16_t('-'));
-    }
-
-    // Use uppercase for country part, e.g. "en-US", not "en-us", see BCP47
-    // only uppercase 2-letter country codes, not "zh-Hant", "de-DE-x-goethe".
-    // NOTE: we should probably rely on the pref being set correctly.
-    if (lang.Length() > 2) {
-      nsCharSeparatedTokenizer localeTokenizer(lang, '-');
-      int32_t pos = 0;
-      bool first = true;
-      while (localeTokenizer.hasMoreTokens()) {
-        const nsSubstring& code = localeTokenizer.nextToken();
-
-        if (code.Length() == 2 && !first) {
-          nsAutoString upper(code);
-          ToUpperCase(upper);
-          lang.Replace(pos, code.Length(), upper);
-        }
-
-        pos += code.Length() + 1; // 1 is the separator
-        first = false;
-      }
-    }
-
-    aLanguages.AppendElement(lang);
-  }
-}
-
-/**
- * Do not use UI language (chosen app locale) here but the first value set in
- * the Accept Languages header, see ::GetAcceptLanguages().
- *
- * See RFC 2616, Section 15.1.4 "Privacy Issues Connected to Accept Headers" for
- * the reasons why.
- */
-NS_IMETHODIMP
-Navigator::GetLanguage(nsAString& aLanguage)
-{
-  nsTArray<nsString> languages;
-  GetLanguages(languages);
-  if (languages.Length() >= 1) {
-    aLanguage.Assign(languages[0]);
-  } else {
-    aLanguage.Truncate();
+  // Checks and fixups:
+  // replace "_" with "-" to avoid POSIX/Windows "en_US" notation.
+  if (aLanguage.Length() > 2 && aLanguage[2] == char16_t('_')) {
+    aLanguage.Replace(2, 1, char16_t('-')); // TODO replace all
   }
 
+  // Use uppercase for country part, e.g. "en-US", not "en-us", see BCP47
+  // only uppercase 2-letter country codes, not "zh-Hant", "de-DE-x-goethe".
+  if (aLanguage.Length() <= 2) {
     return NS_OK;
-}
+  }
 
-void
-Navigator::GetLanguages(nsTArray<nsString>& aLanguages)
-{
-  GetAcceptLanguages(aLanguages);
+  nsCharSeparatedTokenizer localeTokenizer(aLanguage, '-');
+  int32_t pos = 0;
+  bool first = true;
+  while (localeTokenizer.hasMoreTokens()) {
+    const nsSubstring& code = localeTokenizer.nextToken();
 
-  // The returned value is cached by the binding code. The window listen to the
-  // accept languages change and will clear the cache when needed. It has to
-  // take care of dispatching the DOM event already and the invalidation and the
-  // event has to be timed correctly.
+    if (code.Length() == 2 && !first) {
+      nsAutoString upper(code);
+      ToUpperCase(upper);
+      aLanguage.Replace(pos, code.Length(), upper);
+    }
+
+    pos += code.Length() + 1; // 1 is the separator
+    first = false;
+  }
+
+  return NS_OK;
 }
 
 NS_IMETHODIMP
diff --git a/dom/base/Navigator.h b/dom/base/Navigator.h
index 0fef4cb8afc..c586b2e0561 100644
--- a/dom/base/Navigator.h
+++ b/dom/base/Navigator.h
@@ -251,8 +251,6 @@ public:
                     JS::MutableHandle<JSPropertyDescriptor> aDesc);
   void GetOwnPropertyNames(JSContext* aCx, nsTArray<nsString>& aNames,
                            ErrorResult& aRv);
-  void GetLanguages(nsTArray<nsString>& aLanguages);
-  void GetAcceptLanguages(nsTArray<nsString>& aLanguages);
 
   // WebIDL helper methods
   static bool HasBatterySupport(JSContext* /* unused*/, JSObject* /*unused */);
diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index c42670bffbd..a9e1f502a88 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -218,7 +218,6 @@
 #include "nsITabChild.h"
 #include "mozilla/dom/MediaQueryList.h"
 #include "mozilla/dom/ScriptSettings.h"
-#include "mozilla/dom/NavigatorBinding.h"
 #ifdef HAVE_SIDEBAR
 #include "mozilla/dom/ExternalBinding.h"
 #endif
@@ -1138,8 +1137,6 @@ nsGlobalWindow::nsGlobalWindow(nsGlobalWindow *aOuterWindow)
         // events. Use a strong reference.
         os->AddObserver(mObserver, "dom-storage2-changed", false);
       }
-
-      Preferences::AddStrongObserver(mObserver, "intl.accept_languages");
     }
   } else {
     // |this| is an outer window. Outer windows start out frozen and
@@ -1422,8 +1419,6 @@ nsGlobalWindow::CleanUp()
       mIdleService->RemoveIdleObserver(mObserver, MIN_IDLE_NOTIFICATION_TIME_S);
     }
 
-    Preferences::RemoveObserver(mObserver, "intl.accept_languages");
-
     // Drop its reference to this dying window, in case for some bogus reason
     // the object stays around.
     mObserver->Forget();
@@ -11213,32 +11208,6 @@ nsGlobalWindow::Observe(nsISupports* aSubject, const char* aTopic,
   }
 #endif // MOZ_B2G
 
-  if (!nsCRT::strcmp(aTopic, NS_PREFBRANCH_PREFCHANGE_TOPIC_ID)) {
-    MOZ_ASSERT(!nsCRT::strcmp(aData, "intl.accept_languages"));
-    MOZ_ASSERT(IsInnerWindow());
-
-    // The user preferred languages have changed, we need to fire an event on
-    // Window object and invalidate the cache for navigator.languages. It is
-    // done for every change which can be a waste of cycles but those should be
-    // fairly rare.
-    // We MUST invalidate navigator.languages before sending the event in the
-    // very likely situation where an event handler will try to read its value.
-
-    if (mNavigator) {
-      NavigatorBinding::ClearCachedLanguagesValue(mNavigator);
-    }
-
-    nsCOMPtr<nsIDOMEvent> event;
-    NS_NewDOMEvent(getter_AddRefs(event), this, nullptr, nullptr);
-    nsresult rv = event->InitEvent(NS_LITERAL_STRING("languagechange"), false, false);
-    NS_ENSURE_SUCCESS(rv, rv);
-
-    event->SetTrusted(true);
-
-    bool dummy;
-    return DispatchEvent(event, &dummy);
-  }
-
   NS_WARNING("unrecognized topic in nsGlobalWindow::Observe");
   return NS_ERROR_FAILURE;
 }
diff --git a/dom/base/test/mochitest.ini b/dom/base/test/mochitest.ini
index 6f431c9f543..e77a9d1db63 100644
--- a/dom/base/test/mochitest.ini
+++ b/dom/base/test/mochitest.ini
@@ -44,7 +44,6 @@ support-files =
 [test_messageChannel_unshipped.html]
 [test_named_frames.html]
 [test_navigator_resolve_identity.html]
-[test_navigator_language.html]
 [test_nondomexception.html]
 [test_openDialogChromeOnly.html]
 [test_postMessage_solidus.html]
diff --git a/dom/base/test/test_navigator_language.html b/dom/base/test/test_navigator_language.html
deleted file mode 100644
index f468cb1b1c6..00000000000
--- a/dom/base/test/test_navigator_language.html
+++ /dev/null
@@ -1,227 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=889335
--->
-<head>
-  <meta charset="utf-8">
-  <title>Test for NavigatorLanguage</title>
-  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=889335">Mozilla Bug 889335</a>
-<p id="display"></p>
-<div id="content" style="display: none">
-</div>
-<pre id="test">
-</pre>
-<script type="application/javascript;version=1.7">
-  "use strict";
-
-  SimpleTest.waitForExplicitFinish();
-
-  /** Test for NavigatorLanguage **/
-  var prefValue = null;
-  var actualLanguageChangesFromHandler = 0;
-  var actualLanguageChangesFromAVL = 0;
-  var expectedLanguageChanges = 0;
-
-  function setUp() {
-    try {
-      prefValue = SpecialPowers.getCharPref('intl.accept_languages');
-    } catch (e) {
-    }
-  }
-
-  function tearDown() {
-    SpecialPowers.setCharPref('intl.accept_languages', prefValue);
-  }
-
-  var testValues = [
-    { accept_languages: 'foo', language: 'foo', languages: ['foo'] },
-    { accept_languages: '', language: '', languages: [] },
-    { accept_languages: 'foo,bar', language: 'foo', languages: [ 'foo', 'bar' ] },
-    { accept_languages: '  foo , bar ', language: 'foo', languages: [ 'foo', 'bar' ] },
-    { accept_languages: '  foo ; bar ', language: 'foo ; bar', languages: [ 'foo ; bar' ] },
-    { accept_languages: '_foo_', language: '_foo_', languages: ['_foo_'] },
-    { accept_languages: 'en_', language: 'en-', languages: ['en-'] },
-    { accept_languages: 'en__', language: 'en-_', languages: ['en-_'] },
-    { accept_languages: 'en_US, fr_FR', language: 'en-US', languages: ['en-US', 'fr-FR'] },
-    { accept_languages: 'en_US_CA', language: 'en-US_CA', languages: ['en-US_CA'] },
-    { accept_languages: 'en_us-ca', language: 'en-US-CA', languages: ['en-US-CA'] },
-    { accept_languages: 'en_us-cal, en_us-c', language: 'en-US-cal', languages: ['en-US-cal', 'en-US-c'] },
-  ];
-
-  var currentTestIdx = 0;
-  var tests = [];
-  function nextTest() {
-    currentTestIdx++;
-    if (currentTestIdx >= tests.length) {
-      tearDown();
-      SimpleTest.finish();
-    }
-
-    tests[currentTestIdx]();
-  }
-
-  // Check that the API is there.
-  tests.push(function testAPIPresence() {
-    ok('language' in window.navigator);
-    ok('languages' in window.navigator);
-    ok('onlanguagechange' in window);
-
-    nextTest();
-  });
-
-  // Check that calling navigator.languages return the same array, unless there
-  // was a change.
-  tests.push(function testArrayCached() {
-    var previous = navigator.languages;
-    is(navigator.languages, navigator.languages, "navigator.languages is cached");
-    is(navigator.languages, previous, "navigator.languages is cached");
-
-    window.onlanguagechange = function() {
-      isnot(navigator.languages, previous, "navigator.languages cached value was updated");
-      window.onlanguagechange = null;
-
-      nextTest();
-    }
-
-    setTimeout(function() {
-      SpecialPowers.setCharPref('intl.accept_languages', 'testArrayCached');
-    }, 0);
-  });
-
-  // Test that event handler inside the <body> works as expected and that the
-  // event has the expected properties.
-  tests.push(function testEventProperties() {
-    document.body.setAttribute('onlanguagechange',
-      "document.body.removeAttribute('onlanguagechange');" +
-      "is(event.cancelable, false); is(event.bubbles, false);" +
-      "nextTest();");
-
-    setTimeout(function() {
-      SpecialPowers.setCharPref('intl.accept_languages', 'testEventProperties');
-    }, 0);
-  });
-
-  // Check that the returned values such as the behavior when the underlying
-  // languages change.
-  tests.push(function testBasicBehaviour() {
-    function checkIfDoneAndProceed() {
-      if (actualLanguageChangesFromHandler == actualLanguageChangesFromAVL) {
-        if (genEvents.next().done) {
-          window.onlanguagechange = null;
-          window.removeEventListener('languagechange', languageChangeAVL);
-          nextTest();
-        }
-      }
-    }
-    window.onlanguagechange = function() {
-      actualLanguageChangesFromHandler++;
-      checkIfDoneAndProceed();
-    }
-    function languageChangeAVL() {
-      actualLanguageChangesFromAVL++;
-      checkIfDoneAndProceed();
-    }
-    window.addEventListener('languagechange', languageChangeAVL);
-
-    function* testEvents() {
-      for (var i = 0; i < testValues.length; ++i) {
-        var data = testValues[i];
-        setTimeout(function(data) {
-          SpecialPowers.setCharPref('intl.accept_languages', data.accept_languages);
-        }, 0, data);
-        expectedLanguageChanges++;
-        yield undefined;
-
-        is(actualLanguageChangesFromAVL, expectedLanguageChanges);
-        is(actualLanguageChangesFromHandler, expectedLanguageChanges);
-
-        is(navigator.language, data.language);
-        is(navigator.languages.length, data.languages.length);
-        if (navigator.languages.length > 0) {
-          is(navigator.languages[0], navigator.language)
-        }
-        for (var j = 0; j < navigator.languages.length; ++j) {
-          is(navigator.languages[j], data.languages[j]);
-        }
-      }
-    }
-
-    var genEvents = testEvents();
-    genEvents.next();
-  });
-
-  // Check that the orientationchange event isn't sent twice if the preference
-  // is set to the same value.
-  tests.push(function testOnlyFireIfRealChange() {
-    function* changeLanguage() {
-      setTimeout(function() {
-        SpecialPowers.setCharPref('intl.accept_languages', 'fr-CA');
-      });
-      yield undefined;
-
-      setTimeout(function() {
-        // Twice the same change, should fire only one event.
-        SpecialPowers.setCharPref('intl.accept_languages', 'fr-CA');
-        setTimeout(function() {
-          // A real change to tell the test it should now count how many changes were
-          // received until now.
-          SpecialPowers.setCharPref('intl.accept_languages', 'fr-FR');
-        });
-      });
-      yield undefined;
-    }
-
-    var genChanges = changeLanguage();
-
-    var doubleEventCount = 0;
-    window.onlanguagechange = function() {
-      if (navigator.language == 'fr-FR') {
-        is(1, doubleEventCount);
-        window.onlanguagechange = null;
-        nextTest();
-        return;
-      }
-
-      if (navigator.language == 'fr-CA') {
-        doubleEventCount++;
-      }
-      genChanges.next();
-    }
-
-    genChanges.next();
-  });
-
-  // Check that there is no crash when a change happen after a window listening
-  // to them is killed.
-  tests.push(function testThatAddingAnEventDoesNotHaveSideEffects() {
-    var frame = document.createElement('iframe');
-    frame.src = 'data:text/html,<script>window.onlanguagechange=function(){}<\/script>';
-    document.body.appendChild(frame);
-
-    frame.contentWindow.onload = function() {
-      document.body.removeChild(frame);
-      frame = null;
-
-      SpecialPowers.exactGC(window, function() {
-        // This should not crash.
-        SpecialPowers.setCharPref('intl.accept_languages', 'en-GB');
-
-        nextTest();
-      });
-    }
-  });
-
-  // There is one test using document.body.
-  addLoadEvent(function() {
-    setUp();
-    tests[0]();
-  });
-
-</script>
-</body>
-</html>
diff --git a/dom/events/EventNameList.h b/dom/events/EventNameList.h
index 83702a6dfe8..cbb6f54efda 100644
--- a/dom/events/EventNameList.h
+++ b/dom/events/EventNameList.h
@@ -464,10 +464,6 @@ WINDOW_EVENT(hashchange,
              NS_HASHCHANGE,
              EventNameType_XUL | EventNameType_HTMLBodyOrFramesetOnly,
              NS_EVENT)
-WINDOW_EVENT(languagechange,
-             NS_LANGUAGECHANGE,
-             EventNameType_HTMLBodyOrFramesetOnly,
-             NS_EVENT)
 // XXXbz Should the onmessage attribute on <body> really not work?  If so, do we
 // need a different macro to flag things like that (IDL, but not content
 // attributes on body/frameset), or is just using EventNameType_None enough?
diff --git a/dom/interfaces/base/nsIDOMWindow.idl b/dom/interfaces/base/nsIDOMWindow.idl
index e8d844ea670..a66afbe08cc 100644
--- a/dom/interfaces/base/nsIDOMWindow.idl
+++ b/dom/interfaces/base/nsIDOMWindow.idl
@@ -24,7 +24,7 @@ interface nsIVariant;
  * @see <http://www.whatwg.org/html/#window>
  */
 
-[scriptable, uuid(fbefa573-0ba2-4d15-befb-d60277643a0b)]
+[scriptable, uuid(8c115ab3-cf96-492c-850c-3b18056b45e2)]
 interface nsIDOMWindow : nsISupports
 {
   // the current browsing context
@@ -478,7 +478,6 @@ interface nsIDOMWindow : nsISupports
   [implicit_jscontext] attribute jsval onbeforeprint;
   [implicit_jscontext] attribute jsval onbeforeunload;
   [implicit_jscontext] attribute jsval onhashchange;
-  [implicit_jscontext] attribute jsval onlanguagechange;
   [implicit_jscontext] attribute jsval onmessage;
   [implicit_jscontext] attribute jsval onoffline;
   [implicit_jscontext] attribute jsval ononline;
diff --git a/dom/webidl/EventHandler.webidl b/dom/webidl/EventHandler.webidl
index 5ad29bec157..216ddd686da 100644
--- a/dom/webidl/EventHandler.webidl
+++ b/dom/webidl/EventHandler.webidl
@@ -123,7 +123,6 @@ interface WindowEventHandlers {
            attribute EventHandler onbeforeprint;
            attribute OnBeforeUnloadEventHandler onbeforeunload;
            attribute EventHandler onhashchange;
-           attribute EventHandler onlanguagechange;
            attribute EventHandler onmessage;
            attribute EventHandler onoffline;
            attribute EventHandler ononline;
diff --git a/dom/webidl/Navigator.webidl b/dom/webidl/Navigator.webidl
index 79b4c939abe..0634130a845 100644
--- a/dom/webidl/Navigator.webidl
+++ b/dom/webidl/Navigator.webidl
@@ -52,7 +52,6 @@ interface NavigatorID {
 [NoInterfaceObject]
 interface NavigatorLanguage {
   readonly attribute DOMString? language;
-  [Pure, Cached, Frozen] readonly attribute sequence<DOMString> languages;
 };
 
 [NoInterfaceObject]
diff --git a/widget/BasicEvents.h b/widget/BasicEvents.h
index 0f4ec9102f8..1505217502e 100644
--- a/widget/BasicEvents.h
+++ b/widget/BasicEvents.h
@@ -120,8 +120,6 @@ enum nsEventStructType
 // HiDPI mode.
 #define NS_PLUGIN_RESOLUTION_CHANGED     (NS_WINDOW_START + 69)
 
-#define NS_LANGUAGECHANGE                (NS_WINDOW_START + 70)
-
 #define NS_MOUSE_MESSAGE_START          300
 #define NS_MOUSE_MOVE                   (NS_MOUSE_MESSAGE_START)
 #define NS_MOUSE_BUTTON_UP              (NS_MOUSE_MESSAGE_START + 1)