diff --git a/toolkit/components/extensions/Extension.jsm b/toolkit/components/extensions/Extension.jsm
index ee1562a6e251..ff4319903db3 100644
--- a/toolkit/components/extensions/Extension.jsm
+++ b/toolkit/components/extensions/Extension.jsm
@@ -1685,7 +1685,7 @@ this.Langpack = class extends ExtensionData {
 
     // Check if there's a root directory `/localization` in the langpack.
     // If there is one, add it with the name `toolkit` as a FileSource.
-    const entries = await this.readDirectory("./localization");
+    const entries = await this.readDirectory("localization");
     if (entries.length > 0) {
       l10nRegistrySources.toolkit = "";
     }
@@ -1729,6 +1729,9 @@ this.Langpack = class extends ExtensionData {
         `resource://${this.langpackId}/${basePath}localization/{locale}/`
       ));
     }
+
+    Services.obs.notifyObservers({wrappedJSObject: {langpack: this}},
+                                 "webextension-langpack-startup");
   }
 
   async shutdown(reason) {
diff --git a/toolkit/mozapps/extensions/test/addons/langpack_1/browser/localization/und/browser.ftl b/toolkit/mozapps/extensions/test/addons/langpack_1/browser/localization/und/browser.ftl
new file mode 100644
index 000000000000..e1ee61f7b31b
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/addons/langpack_1/browser/localization/und/browser.ftl
@@ -0,0 +1 @@
+message-browser = Value from Browser
diff --git a/toolkit/mozapps/extensions/test/addons/langpack_1/chrome/und/locale/und/global/test.properties b/toolkit/mozapps/extensions/test/addons/langpack_1/chrome/und/locale/und/global/test.properties
new file mode 100644
index 000000000000..509cb29e4745
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/addons/langpack_1/chrome/und/locale/und/global/test.properties
@@ -0,0 +1 @@
+message = Value from .properties
diff --git a/toolkit/mozapps/extensions/test/addons/langpack_1/localization/und/toolkit_test.ftl b/toolkit/mozapps/extensions/test/addons/langpack_1/localization/und/toolkit_test.ftl
new file mode 100644
index 000000000000..643abd059d04
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/addons/langpack_1/localization/und/toolkit_test.ftl
@@ -0,0 +1 @@
+message-id1 = Value 1
diff --git a/toolkit/mozapps/extensions/test/addons/langpack_1/manifest.json b/toolkit/mozapps/extensions/test/addons/langpack_1/manifest.json
new file mode 100644
index 000000000000..d2b0c9b1ba3a
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/addons/langpack_1/manifest.json
@@ -0,0 +1,29 @@
+{
+  "name": "und Language Pack",
+  "version": "1.0",
+  "manifest_version": 2,
+  "applications": {
+    "gecko": {
+      "id": "langpack-und@test.mozilla.org",
+      "strict_min_version": "58.0",
+      "strict_max_version": "58.*"
+    }
+  },
+  "sources": {
+    "browser": {
+      "base_path": "browser/"
+    }
+  },
+  "langpack_id": "und",
+  "version": "1",
+  "languages": {
+    "und": {
+      "chrome_resources": {
+        "global": "chrome/und/locale/und/global/"
+      },
+      "version": "20171001190118"
+    }
+  },
+  "author": "Mozilla Localization Task Force",
+  "description": "Language pack for Testy for und"
+}
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_webextension_langpack.js b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_langpack.js
new file mode 100644
index 000000000000..8c0873563af7
--- /dev/null
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_langpack.js
@@ -0,0 +1,131 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+Components.utils.import("resource://gre/modules/AppConstants.jsm");
+
+const { Services } = Components.utils.import("resource://gre/modules/Services.jsm", {});
+const { L10nRegistry } = Components.utils.import("resource://gre/modules/L10nRegistry.jsm", {});
+
+const ID = "langpack-und@test.mozilla.org";
+
+const profileDir = gProfD.clone();
+profileDir.append("extensions");
+
+createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "58");
+startupManager();
+
+function promiseLangpackStartup() {
+  return new Promise(resolve => {
+    const EVENT = "webextension-langpack-startup";
+    Services.obs.addObserver(function observer() {
+      Services.obs.removeObserver(observer, EVENT);
+      resolve();
+    }, EVENT);
+  });
+}
+
+/**
+ * This is a basic life-cycle test which verifies that
+ * the language pack registers and unregisters correct
+ * languages at various stages.
+ */
+add_task(async function() {
+  // Make sure that `und` locale is not installed.
+  equal(L10nRegistry.getAvailableLocales().includes("und"), false);
+  equal(Services.locale.getAvailableLocales().includes("und"), false);
+
+  let [{addon}, ] = await Promise.all([
+    promiseInstallFile(do_get_addon("langpack_1"), true),
+    promiseLangpackStartup()
+  ]);
+
+  // Now make sure that `und` locale is available.
+  equal(L10nRegistry.getAvailableLocales().includes("und"), true);
+  equal(Services.locale.getAvailableLocales().includes("und"), true);
+
+  addon.userDisabled = true;
+
+  // It is not available after the langpack has been disabled.
+  equal(L10nRegistry.getAvailableLocales().includes("und"), false);
+  equal(Services.locale.getAvailableLocales().includes("und"), false);
+
+  addon.userDisabled = false;
+  await promiseLangpackStartup();
+
+  // After re-enabling it, the `und` locale is available again.
+  equal(L10nRegistry.getAvailableLocales().includes("und"), true);
+  equal(Services.locale.getAvailableLocales().includes("und"), true);
+
+  addon.uninstall();
+
+  // After the langpack has been uninstalled, no more `und` in locales.
+  equal(L10nRegistry.getAvailableLocales().includes("und"), false);
+  equal(Services.locale.getAvailableLocales().includes("und"), false);
+});
+
+/**
+ * This test verifies that registries are able to load and return
+ * correct strings available in the language pack.
+ */
+add_task(async function() {
+  let [{addon}, ] = await Promise.all([
+    promiseInstallFile(do_get_addon("langpack_1"), true),
+    promiseLangpackStartup()
+  ]);
+
+  {
+    // Toolkit string
+    let ctxs = L10nRegistry.generateContexts(["und"], ["toolkit_test.ftl"]);
+    let ctx0 = (await ctxs.next()).value;
+    equal(ctx0.hasMessage("message-id1"), true);
+  }
+
+  {
+    // Browser string
+    let ctxs = L10nRegistry.generateContexts(["und"], ["browser.ftl"]);
+    let ctx0 = (await ctxs.next()).value;
+    equal(ctx0.hasMessage("message-browser"), true);
+  }
+
+  {
+    // Test chrome package
+    let reqLocs = Services.locale.getRequestedLocales();
+    Services.locale.setRequestedLocales(["und"]);
+
+    let bundle = Services.strings.createBundle(
+      "chrome://global/locale/test.properties"
+    );
+    let entry = bundle.GetStringFromName("message");
+    equal(entry, "Value from .properties");
+
+    Services.locale.setRequestedLocales(reqLocs);
+  }
+
+  addon.uninstall();
+});
+
+/**
+ * This test verifies that language pack will get disabled after app
+ * gets upgraded.
+ */
+add_task(async function() {
+  let [{addon}, ] = await Promise.all([
+    promiseInstallFile(do_get_addon("langpack_1"), true),
+    promiseLangpackStartup()
+  ]);
+  do_check_true(addon.isActive);
+
+  await promiseShutdownManager();
+
+  gAppInfo.version = "59";
+  gAppInfo.platformVersion = "59";
+
+  await promiseStartupManager(true);
+
+  addon = await promiseAddonByID(ID);
+  do_check_false(addon.isActive);
+  do_check_true(addon.appDisabled);
+
+  addon.uninstall();
+});
diff --git a/toolkit/mozapps/extensions/test/xpcshell/xpcshell-shared.ini b/toolkit/mozapps/extensions/test/xpcshell/xpcshell-shared.ini
index 5d51609ea5ef..56300c997320 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/xpcshell-shared.ini
+++ b/toolkit/mozapps/extensions/test/xpcshell/xpcshell-shared.ini
@@ -331,6 +331,9 @@ tags = webextensions
 [test_webextension_embedded.js]
 skip-if = appname == "thunderbird"
 tags = webextensions
+[test_webextension_langpack.js]
+skip-if = appname == "thunderbird"
+tags = webextensions
 [test_webextension_install_syntax_error.js]
 skip-if = appname == "thunderbird"
 tags = webextensions