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diff --git a/toolkit/jetpack/sdk/l10n/locale.js b/toolkit/jetpack/sdk/l10n/locale.js
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+++ b/toolkit/jetpack/sdk/l10n/locale.js
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+module.metadata = {
+  "stability": "unstable"
+};
+
+const prefs = require("../preferences/service");
+const { Cu, Cc, Ci } = require("chrome");
+const { Services } = Cu.import("resource://gre/modules/Services.jsm");
+
+/**
+ * Gets the currently selected locale for display.
+ * Gets all usable locale that we can use sorted by priority of relevance
+ * @return  Array of locales, begins with highest priority
+ */
+const PREF_MATCH_OS_LOCALE  = "intl.locale.matchOS";
+const PREF_SELECTED_LOCALE  = "general.useragent.locale";
+const PREF_ACCEPT_LANGUAGES = "intl.accept_languages";
+
+function getPreferedLocales(caseSensitve) {
+  let locales = [];
+  function addLocale(locale) {
+    locale = locale.trim();
+    if (!caseSensitve)
+      locale = locale.toLowerCase();
+    if (locales.indexOf(locale) === -1)
+      locales.push(locale);
+  }
+
+  // Most important locale is OS one. But we use it, only if
+  // "intl.locale.matchOS" pref is set to `true`.
+  // Currently only used for multi-locales mobile builds.
+  // http://mxr.mozilla.org/mozilla-central/source/mobile/android/installer/Makefile.in#46
+  if (prefs.get(PREF_MATCH_OS_LOCALE, false)) {
+    let localeService = Cc["@mozilla.org/intl/nslocaleservice;1"].
+                        getService(Ci.nsILocaleService);
+    let osLocale = localeService.getLocaleComponentForUserAgent();
+    addLocale(osLocale);
+  }
+
+  // In some cases, mainly on Fennec and on Linux version,
+  // `general.useragent.locale` is a special 'localized' value, like:
+  // "chrome://global/locale/intl.properties"
+  let browserUiLocale = prefs.getLocalized(PREF_SELECTED_LOCALE, "") ||
+                        prefs.get(PREF_SELECTED_LOCALE, "");
+  if (browserUiLocale)
+    addLocale(browserUiLocale);
+
+  // Third priority is the list of locales used for web content
+  let contentLocales = prefs.getLocalized(PREF_ACCEPT_LANGUAGES, "") ||
+                       prefs.get(PREF_ACCEPT_LANGUAGES, "");
+  if (contentLocales) {
+    // This list is a string of locales seperated by commas.
+    // There is spaces after commas, so strip each item
+    for (let locale of contentLocales.split(","))
+      addLocale(locale.replace(/(^\s+)|(\s+$)/g, ""));
+  }
+
+  // Finally, we ensure that en-US is the final fallback if it wasn't added
+  addLocale("en-US");
+
+  return locales;
+}
+exports.getPreferedLocales = getPreferedLocales;
+
+/**
+ * Selects the closest matching locale from a list of locales.
+ *
+ * @param  aLocales
+ *         An array of available locales
+ * @param  aMatchLocales
+ *         An array of prefered locales, ordered by priority. Most wanted first.
+ *         Locales have to be in lowercase.
+ *         If null, uses getPreferedLocales() results
+ * @return the best match for the currently selected locale
+ *
+ * Stolen from http://dxr.mozilla.org/mozilla-central/source/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+ */
+exports.findClosestLocale = function findClosestLocale(aLocales, aMatchLocales) {
+  aMatchLocales = aMatchLocales || getPreferedLocales();
+
+  // Holds the best matching localized resource
+  let bestmatch = null;
+  // The number of locale parts it matched with
+  let bestmatchcount = 0;
+  // The number of locale parts in the match
+  let bestpartcount = 0;
+
+  for (let locale of aMatchLocales) {
+    let lparts = locale.split("-");
+    for (let localized of aLocales) {
+      let found = localized.toLowerCase();
+      // Exact match is returned immediately
+      if (locale == found)
+        return localized;
+
+      let fparts = found.split("-");
+      /* If we have found a possible match and this one isn't any longer
+         then we dont need to check further. */
+      if (bestmatch && fparts.length < bestmatchcount)
+        continue;
+
+      // Count the number of parts that match
+      let maxmatchcount = Math.min(fparts.length, lparts.length);
+      let matchcount = 0;
+      while (matchcount < maxmatchcount &&
+             fparts[matchcount] == lparts[matchcount])
+        matchcount++;
+
+      /* If we matched more than the last best match or matched the same and
+         this locale is less specific than the last best match. */
+      if (matchcount > bestmatchcount ||
+         (matchcount == bestmatchcount && fparts.length < bestpartcount)) {
+        bestmatch = localized;
+        bestmatchcount = matchcount;
+        bestpartcount = fparts.length;
+      }
+    }
+    // If we found a valid match for this locale return it
+    if (bestmatch)
+      return bestmatch;
+  }
+  return null;
+}