--- a/browser/app/profile/firefox.js
+++ b/browser/app/profile/firefox.js
@@ -1733,8 +1733,11 @@ pref("app.normandy.shieldLearnMoreUrl", 
 pref("app.shield.optoutstudies.enabled", true);
 #else
 pref("app.shield.optoutstudies.enabled", false);
 #endif
 
 // Savant Shield study preferences
 pref("shield.savant.enabled", false);
 pref("shield.savant.loglevel", "warn");
+
+// Multi-lingual preferences
+pref("intl.multilingual.enabled", false);

--- a/browser/components/preferences/in-content/main.js
+++ b/browser/components/preferences/in-content/main.js
@@ -9,16 +9,18 @@
 
 ChromeUtils.import("resource://gre/modules/Services.jsm");
 ChromeUtils.import("resource://gre/modules/Downloads.jsm");
 ChromeUtils.import("resource://gre/modules/FileUtils.jsm");
 ChromeUtils.import("resource:///modules/ShellService.jsm");
 ChromeUtils.import("resource:///modules/TransientPrefs.jsm");
 ChromeUtils.import("resource://gre/modules/AppConstants.jsm");
 ChromeUtils.import("resource://gre/modules/DownloadUtils.jsm");
+ChromeUtils.import("resource://gre/modules/L10nRegistry.jsm");
+ChromeUtils.import("resource://gre/modules/Localization.jsm");
 ChromeUtils.defineModuleGetter(this, "CloudStorage",
   "resource://gre/modules/CloudStorage.jsm");
 
 XPCOMUtils.defineLazyServiceGetters(this, {
   gCategoryManager: ["@mozilla.org/categorymanager;1", "nsICategoryManager"],
   gHandlerService: ["@mozilla.org/uriloader/handler-service;1", "nsIHandlerService"],
   gMIMEService: ["@mozilla.org/mime;1", "nsIMIMEService"],
   gWebContentContentConverterService: ["@mozilla.org/embeddor.implemented/web-content-handler-registrar;1", "nsIWebContentConverterService"],
@@ -239,16 +241,27 @@ if (AppConstants.MOZ_UPDATER) {
     ]);
   }
 }
 
 // A promise that resolves when the list of application handlers is loaded.
 // We store this in a global so tests can await it.
 var promiseLoadHandlersList;
 
+// Load the preferences string bundle for a given locale.
+function getBundleForLocale(locale) {
+  function generateContexts(resourceIds) {
+    return L10nRegistry.generateContexts([locale], resourceIds);
+  }
+  return new Localization([
+    "browser/preferences/preferences.ftl",
+    "branding/brand.ftl",
+  ], generateContexts);
+}
+
 var gNodeToObjectMap = new WeakMap();
 
 var gMainPane = {
   // The set of types the app knows how to handle.  A hash of HandlerInfoWrapper
   // objects, indexed by type.
   _handledTypes: {},
 
   // The list of types we can show, sorted by the sort column/direction.
@@ -344,16 +357,20 @@ var gMainPane = {
 
     let connectionSettingsLink = document.getElementById("connectionSettingsLearnMore");
     let connectionSettingsUrl = Services.urlFormatter.formatURLPref("app.support.baseURL") +
                                 "prefs-connection-settings";
     connectionSettingsLink.setAttribute("href", connectionSettingsUrl);
     this.updateProxySettingsUI();
     initializeProxyUI(gMainPane);
 
+    if (Services.prefs.getBoolPref("intl.multilingual.enabled")) {
+      gMainPane.initBrowserLocale();
+    }
+
     if (AppConstants.platform == "win") {
       // Functionality for "Show tabs in taskbar" on Windows 7 and up.
       try {
         let ver = parseFloat(Services.sysinfo.getProperty("version"));
         let showTabsInTaskbar = document.getElementById("showTabsInTaskbar");
         showTabsInTaskbar.hidden = ver < 6.1;
       } catch (ex) { }
     }
@@ -754,16 +771,70 @@ var gMainPane = {
       checkbox.removeAttribute("disabled");
       newValue = startupPref.value === this.STARTUP_PREF_RESTORE_SESSION;
     }
     if (checkbox.checked !== newValue) {
       checkbox.checked = newValue;
     }
   },
 
+  initBrowserLocale() {
+    let localeCodes = Services.locale.getAvailableLocales();
+    let localeNames = Services.intl.getLocaleDisplayNames(undefined, localeCodes);
+    let locales = localeCodes.map((code, i) => ({code, name: localeNames[i]}));
+    locales.sort((a, b) => a.name > b.name);
+
+    let fragment = document.createDocumentFragment();
+    for (let {code, name} of locales) {
+      let menuitem = document.createElement("menuitem");
+      menuitem.setAttribute("value", code);
+      menuitem.setAttribute("label", name);
+      fragment.appendChild(menuitem);
+    }
+    let menulist = document.getElementById("defaultBrowserLanguage");
+    let menupopup = menulist.querySelector("menupopup");
+    menupopup.appendChild(fragment);
+    menulist.value = Services.locale.getRequestedLocale();
+
+    document.getElementById("browserLanguagesBox").hidden = false;
+  },
+
+  /* Show the confirmation message bar to allow a restart into the new language. */
+  async onBrowserLanguageChange(event) {
+    let locale = event.target.value;
+    let messageBar = document.getElementById("confirmBrowserLanguage");
+    if (locale == Services.locale.getRequestedLocale()) {
+      messageBar.hidden = true;
+      return;
+    }
+    // Set the text in the message bar for the new locale.
+    let newBundle = getBundleForLocale(locale);
+    let description = messageBar.querySelector("description");
+    description.textContent = await newBundle.formatValue(
+      "confirm-browser-language-change-description");
+    let button = messageBar.querySelector("button");
+    button.setAttribute(
+      "label", await newBundle.formatValue(
+        "confirm-browser-language-change-button"));
+    messageBar.hidden = false;
+  },
+
+  /* Confirm the locale change and restart the browser in the new locale. */
+  confirmBrowserLanguageChange() {
+    let locale = document.getElementById("defaultBrowserLanguage").value;
+    Services.locale.setRequestedLocales([locale]);
+
+    // Restart with the new locale.
+    let cancelQuit = Cc["@mozilla.org/supports-PRBool;1"].createInstance(Ci.nsISupportsPRBool);
+    Services.obs.notifyObservers(cancelQuit, "quit-application-requested", "restart");
+    if (!cancelQuit.data) {
+      Services.startup.quit(Services.startup.eAttemptQuit | Services.startup.eRestart);
+    }
+  },
+
   onBrowserRestoreSessionChange(event) {
     const value = event.target.checked;
     const startupPref = Preferences.get("browser.startup.page");
     let newValue;
 
     if (value) {
       // We need to restore the blank homepage setting in our other pref
       if (startupPref.value === this.STARTUP_PREF_BLANK) {

--- a/browser/components/preferences/in-content/main.xul
+++ b/browser/components/preferences/in-content/main.xul
@@ -277,16 +277,30 @@
     </hbox>
   </vbox>
 </groupbox>
 
 <!-- Languages -->
 <groupbox id="languagesGroup" data-category="paneGeneral" hidden="true">
   <caption><label data-l10n-id="language-header"/></caption>
 
+  <vbox id="browserLanguagesBox" align="start" hidden="true">
+    <description flex="1" controls="chooseBrowserLanguage" data-l10n-id="choose-browser-language-description"/>
+    <menulist id="defaultBrowserLanguage" class="accessory-button" oncommand="gMainPane.onBrowserLanguageChange(event)" flex="1">
+      <menupopup/>
+    </menulist>
+  </vbox>
+  <hbox id="confirmBrowserLanguage" class="message-bar" align="center" hidden="true">
+    <image class="message-bar-icon"/>
+    <hbox class="message-bar-content" align="center" flex="1">
+      <description flex="1"/>
+      <button class="message-bar-button" oncommand="gMainPane.confirmBrowserLanguageChange()"/>
+    </hbox>
+  </hbox>
+
   <hbox id="languagesBox" align="center">
     <description flex="1" control="chooseLanguage" data-l10n-id="choose-language-description"/>
     <!-- Please don't remove the wrapping hbox/vbox/box for these elements. It's used to properly compute the search tooltip position. -->
     <hbox>
       <button id="chooseLanguage"
               class="accessory-button"
               data-l10n-id="choose-button"
               search-l10n-ids="

--- a/browser/locales/en-US/browser/preferences/preferences.ftl
+++ b/browser/locales/en-US/browser/preferences/preferences.ftl
@@ -246,16 +246,20 @@ colors-settings =
 language-header = Language
 
 choose-language-description = Choose your preferred language for displaying pages
 
 choose-button =
     .label = Choose…
     .accesskey = o
 
+choose-browser-language-description = Choose the languages used to display menus, messages, and notifications from { -brand-short-name }.
+confirm-browser-language-change-description = Restart { -brand-short-name } to apply these changes
+confirm-browser-language-change-button = Apply and Restart
+
 translate-web-pages =
     .label = Translate web content
     .accesskey = T
 
 # The <img> element is replaced by the logo of the provider
 # used to provide machine translations for web pages.
 translate-attribution = Translations by <img data-l10n-name="logo"/>
 

--- a/browser/themes/shared/incontentprefs/preferences.inc.css
+++ b/browser/themes/shared/incontentprefs/preferences.inc.css
@@ -898,8 +898,12 @@ menulist[indicator=true] > menupopup men
   visibility: hidden;
 }
 
 /* Proxy port input */
 
 .proxy-port-input {
   width: calc(5ch + 22px); /* 5 chars + 11px padding on both sides */
 }
+
+#defaultBrowserLanguage {
+  min-width: 20em;
+}

--- a/toolkit/themes/shared/in-content/common.inc.css
+++ b/toolkit/themes/shared/in-content/common.inc.css
@@ -36,16 +36,21 @@
   --in-content-link-color-active: #003eaa;
   --in-content-link-color-visited: #0a8dff;
   --in-content-primary-button-background: #0a84ff;
   --in-content-primary-button-background-hover: #0060df;
   --in-content-primary-button-background-active: #003EAA;
   --in-content-table-background: #ebebeb;
   --in-content-table-border-dark-color: #d1d1d1;
   --in-content-table-header-background: #0a84ff;
+  --grey-20: #ededf0;
+  --grey-90: #0c0c0d;
+  --grey-90-a10: rgba(12, 12, 13, 0.1);
+  --grey-90-a20: rgba(12, 12, 13, 0.2);
+  --grey-90-a30: rgba(12, 12, 13, 0.3);
 }
 
 html|html,
 xul|page,
 xul|window {
   font: message-box;
   -moz-appearance: none;
   background-color: var(--in-content-page-background);
@@ -840,8 +845,46 @@ xul|treechildren::-moz-tree-cell-text,
 xul|treechildren::-moz-tree-image {
   color: var(--in-content-text-color);
 }
 
 xul|treechildren::-moz-tree-cell-text(selected),
 xul|treechildren::-moz-tree-image(selected) {
   color: var(--in-content-selected-text);
 }
+
+/* Message bars */
+.message-bar {
+  background-color: var(--grey-20);
+  border-radius: 4px;
+  color: var(--grey-90);
+  min-height: 32px;
+  padding: 0 4px;
+}
+
+/* The message-bar-button styles have extra specificity to override
+ * the defaults for buttons. */
+.message-bar > hbox > .message-bar-button {
+  background-color: var(--grey-90-a10);
+  border: none;
+  border-radius: 2px;
+  height: 24px;
+  margin-inline-start: 8px;
+  padding: 0 8px;
+}
+
+.message-bar > hbox > .message-bar-button:hover {
+  background-color: var(--grey-90-a20);
+}
+
+.message-bar > hbox > .message-bar-button:active {
+  background-color: var(--grey-90-a30);
+}
+
+.message-bar-icon {
+  list-style-image: url("chrome://browser/skin/identity-icon.svg");
+  width: 24px;
+  height: 24px;
+  padding: 4px;
+  margin-inline-end: 4px;
+  -moz-context-properties: fill;
+  fill: currentColor;
+}