--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -45,17 +45,16 @@ XPCOMUtils.defineLazyModuleGetter(this, 
 Cu.import("resource://gre/modules/ExtensionUtils.jsm");
 var {
   runSafeSyncWithoutClone,
   BaseContext,
   LocaleData,
   Messenger,
   injectAPI,
   flushJarCache,
-  detectLanguage,
   getInnerWindowID,
   promiseDocumentReady,
   ChildAPIManager,
   SchemaAPIManager,
 } = ExtensionUtils;
 
 XPCOMUtils.defineLazyGetter(this, "console", ExtensionUtils.getConsole);
 
@@ -150,36 +149,16 @@ var api = context => {
       },
 
       get lastError() {
         return context.lastError;
       },
 
       inIncognitoContext: PrivateBrowsingUtils.isContentWindowPrivate(context.contentWindow),
     },
-
-    i18n: {
-      getMessage: function(messageName, substitutions) {
-        return context.extension.localizeMessage(messageName, substitutions, {cloneScope: context.cloneScope});
-      },
-
-      getAcceptLanguages: function(callback) {
-        let result = context.extension.localeData.acceptLanguages;
-        return context.wrapPromise(Promise.resolve(result), callback);
-      },
-
-      getUILanguage: function() {
-        return context.extension.localeData.uiLocale;
-      },
-
-      detectLanguage: function(text, callback) {
-        let result = detectLanguage(text);
-        return context.wrapPromise(result, callback);
-      },
-    },
   };
 };
 
 // Represents a content script.
 function Script(extension, options, deferred = PromiseUtils.defer()) {
   this.extension = extension;
   this.options = options;
   this.run_at = this.options.run_at;

--- a/toolkit/components/extensions/ext-i18n.js
+++ b/toolkit/components/extensions/ext-i18n.js
@@ -2,17 +2,17 @@
 
 var {classes: Cc, interfaces: Ci, utils: Cu} = Components;
 
 Cu.import("resource://gre/modules/ExtensionUtils.jsm");
 var {
   detectLanguage,
 } = ExtensionUtils;
 
-extensions.registerSchemaAPI("i18n", "addon_parent", context => {
+function i18nApiFactory(context) {
   let {extension} = context;
   return {
     i18n: {
       getMessage: function(messageName, substitutions) {
         return extension.localizeMessage(messageName, substitutions, {cloneScope: context.cloneScope});
       },
 
       getAcceptLanguages: function() {
@@ -24,9 +24,11 @@ extensions.registerSchemaAPI("i18n", "ad
         return extension.localeData.uiLocale;
       },
 
       detectLanguage: function(text) {
         return detectLanguage(text);
       },
     },
   };
-});
+}
+extensions.registerSchemaAPI("i18n", "addon_child", i18nApiFactory);
+extensions.registerSchemaAPI("i18n", "content_child", i18nApiFactory);

--- a/toolkit/components/extensions/extensions-toolkit.manifest
+++ b/toolkit/components/extensions/extensions-toolkit.manifest
@@ -9,25 +9,28 @@ category webextension-scripts i18n chrom
 category webextension-scripts idle chrome://extensions/content/ext-idle.js
 category webextension-scripts webRequest chrome://extensions/content/ext-webRequest.js
 category webextension-scripts webNavigation chrome://extensions/content/ext-webNavigation.js
 category webextension-scripts runtime chrome://extensions/content/ext-runtime.js
 category webextension-scripts extension chrome://extensions/content/ext-extension.js
 category webextension-scripts storage chrome://extensions/content/ext-storage.js
 category webextension-scripts test chrome://extensions/content/ext-test.js
 
+# scripts specific for content process.
+category webextension-scripts-content i18n chrome://extensions/content/ext-i18n.js
+
 # schemas
 category webextension-schemas alarms chrome://extensions/content/schemas/alarms.json
 category webextension-schemas cookies chrome://extensions/content/schemas/cookies.json
 category webextension-schemas downloads chrome://extensions/content/schemas/downloads.json
 category webextension-schemas events chrome://extensions/content/schemas/events.json
 category webextension-schemas extension chrome://extensions/content/schemas/extension.json
 category webextension-schemas extension_types chrome://extensions/content/schemas/extension_types.json
 category webextension-schemas i18n chrome://extensions/content/schemas/i18n.json
 category webextension-schemas idle chrome://extensions/content/schemas/idle.json
 category webextension-schemas management chrome://extensions/content/schemas/management.json
 category webextension-schemas native_host_manifest chrome://extensions/content/schemas/native_host_manifest.json
 category webextension-schemas notifications chrome://extensions/content/schemas/notifications.json
 category webextension-schemas runtime chrome://extensions/content/schemas/runtime.json
 category webextension-schemas storage chrome://extensions/content/schemas/storage.json
 category webextension-schemas test chrome://extensions/content/schemas/test.json
 category webextension-schemas web_navigation chrome://extensions/content/schemas/web_navigation.json
-category webextension-schemas web_request chrome://extensions/content/schemas/web_request.json
\ No newline at end of file
+category webextension-schemas web_request chrome://extensions/content/schemas/web_request.json

--- a/toolkit/components/extensions/schemas/i18n.json
+++ b/toolkit/components/extensions/schemas/i18n.json
@@ -14,16 +14,18 @@
             "optional": "true"
           }
         }
       }
     ]
   },
   {
     "namespace": "i18n",
+    "restrictions": ["content"],
+    "defaultRestrictions": ["content"],
     "description": "Use the <code>browser.i18n</code> infrastructure to implement internationalization across your whole app or extension.",
     "types": [
       {
         "id": "LanguageCode",
         "type": "string",
         "description": "An ISO language code such as <code>en</code> or <code>fr</code>. For a complete list of languages supported by this method, see <a href='http://src.chromium.org/viewvc/chrome/trunk/src/third_party/cld/languages/internal/languages.cc'>kLanguageInfoTable</a>. For an unknown language, <code>und</code> will be returned, which means that [percentage] of the text is unknown to CLD"
       }
     ],