diff -up firefox-51.0/toolkit/mozapps/extensions/internal/XPIProvider.jsm.1005640-accept-lang firefox-51.0/toolkit/mozapps/extensions/internal/XPIProvider.jsm

--- firefox-51.0/toolkit/mozapps/extensions/internal/XPIProvider.jsm.1005640-accept-lang	2017-01-16 17:16:52.000000000 +0100

+++ firefox-51.0/toolkit/mozapps/extensions/internal/XPIProvider.jsm	2017-01-18 12:35:29.380394216 +0100

@@ -2852,6 +2852,11 @@ this.XPIProvider = {

         this.addAddonsToCrashReporter();

       }

+      // Save locale settings to compare it later to check whenever some addon

+      // changed it.

+      var previousLocale = Cc["@mozilla.org/chrome/chrome-registry;1"]

+                          .getService(Ci.nsIXULChromeRegistry).getSelectedLocale("global");

+

       try {

         AddonManagerPrivate.recordTimestamp("XPI_bootstrap_addons_begin");

@@ -2880,6 +2885,14 @@ this.XPIProvider = {

         AddonManagerPrivate.recordException("XPI-BOOTSTRAP", "startup failed", e);

       }

+      var currentLocale = Cc["@mozilla.org/chrome/chrome-registry;1"]

+                         .getService(Ci.nsIXULChromeRegistry).getSelectedLocale("global");

+      if (currentLocale != previousLocale) {

+        // We have to flush string cache if the locale was changed during loading

+        // of addons

+        Services.obs.notifyObservers(null, "chrome-flush-caches", null);

+      }

+

       // Let these shutdown a little earlier when they still have access to most

       // of XPCOM

       Services.obs.addObserver({