8052: Available tab list on Sidebar customize tab is still US contents

after switching to JP region pack. (r=jbetak,sr=mscott)
use "intl.content.langcode" instead of u-a locale

modules/libpref/src/init/all.js

@@ -390,6 +390,7 @@ pref("intl.charsetmenu.composer.cache",     "");
 pref("intl.charsetmenu.browser.cache.size", 5);
 pref("intl.charset.detector",               "chrome://navigator/locale/navigator.properties");
 pref("intl.charset.default",                "chrome://navigator/locale/navigator.properties");
+pref("intl.content.langcode",               "chrome://communicator-region/locale/region.properties");
 pref("font.language.group",                 "chrome://navigator/locale/navigator.properties");
 
 // -- folders (Mac: these are binary aliases.)

xpfe/communicator/resources/locale/en-US/region.properties

@@ -1,3 +1,7 @@
 # search-panel.properties
 #   Localizable URLs
 defaultSearchURL=http://search.netscape.com/cgi-bin/search?charset=UTF-8&search=
+
+# sidebar customize/more... panel locale
+intl.content.langcode=en-US
+

xpfe/components/sidebar/resources/sidebarOverlay.js

@@ -681,7 +681,7 @@ function get_remote_datasource_url() {
       debug("Unable to get remote url pref. What now? "+ex);
     }
     try {
-      locale = prefs.CopyCharPref("intl.content.langcode");
+      locale = prefs.getLocalizedUnicharPref("intl.content.langcode");
     } catch(ex) {
       try {
         debug("No lang code pref, intl.content.langcode.");