Add HelpDialog to the list of domains for the three languages, and make sure that the domain file is loaded (even if the language is changed after the help dialog has been viewed in another language).

unpacked/extensions/HelpDialog.js

@@ -24,7 +24,7 @@
  *  limitations under the License.
  */
 
-(function (HUB,HTML,AJAX,OUTPUT) {
+(function (HUB,HTML,AJAX,OUTPUT,LOCALE) {
 
   var HELP = MathJax.Extension.Help = {
     version: "2.1"
@@ -69,10 +69,14 @@
    *  Handle the Help Dialog box
    */
   HELP.Dialog = function () {
+    LOCALE.loadDomain("HelpDialog",["Post",HELP]);
+  };
+  
+  HELP.Post = function () {
     this.div = MENU.Background(this);
     var help = HTML.addElement(this.div,"div",{
       id: "MathJax_Help"
-    },MathJax.Localization._("HelpDialog",[
+    },LOCALE._("HelpDialog",[
       ["b",{style:{fontSize:"120%"}},[["Help","MathJax Help"]]],
       ["div",{id: "MathJax_HelpContent"},[
         ["p",{},[["MathJax",
@@ -124,7 +128,7 @@
         onclick: HELP.Remove
       }]
     ]));
-    MathJax.Localization.setCSS(help);
+    LOCALE.setCSS(help);
     var doc = (document.documentElement||{});
     var H = window.innerHeight || doc.clientHeight || doc.scrollHeight || 0;
     if (MENU.prototype.msieAboutBug) {
@@ -143,9 +147,8 @@
   MathJax.Callback.Queue(
     HUB.Register.StartupHook("End Config",{}), // wait until config is complete
     ["Styles",AJAX,CONFIG.styles],
-    ["loadDomain",MathJax.Localization,"HelpDialog"],
     ["Post",HUB.Startup.signal,"HelpDialig Ready"],
     ["loadComplete",AJAX,"[MathJax]/extensions/HelpDialog.js"]
   );
 
-})(MathJax.Hub,MathJax.HTML,MathJax.Ajax,MathJax.OutputJax);
+})(MathJax.Hub,MathJax.HTML,MathJax.Ajax,MathJax.OutputJax,MathJax.Localization);

unpacked/localization/de/de.js

@@ -59,6 +59,7 @@ MathJax.Localization.addTranslation("de",null,{
     
     MathMenu: {},
     FontWarnings: {},
+    HelpDialog: {},
     "v1.0-warning": {},
     TeX: {},
     MathML: {},

unpacked/localization/en/en.js

@@ -38,6 +38,7 @@ MathJax.Localization.addTranslation("en",null,{
     },
     MathMenu: {},
     FontWarnings: {},
+    HelpDialog: {},
     "v1.0-warning": {},
     TeX: {},
     MathML: {},

unpacked/localization/fr/fr.js

@@ -38,6 +38,7 @@ MathJax.Localization.addTranslation("fr",null,{
     },
     MathMenu: {},
     FontWarnings: {},
+    HelpDialog: {},
     "v1.0-warning": {},
     TeX: {},
     MathML: {},