Changing the default from "en" to "en_US" to reflect the help file moves.

src/help.php

@@ -119,18 +119,18 @@ if (isset($HTTP_REFERER)) {
 }
   
 if (!isset($squirrelmail_language)) {
-    $squirrelmail_language = 'en';
+    $squirrelmail_language = 'en_US';
 }
 
 if (file_exists("../help/$squirrelmail_language")) {
     $help_exists = true;
     $user_language = $squirrelmail_language;
-} else if (file_exists('../help/en')) {
+} else if (file_exists('../help/en_US')) {
     $help_exists = true;
     echo "<center><font color=\"$color[2]\">";
     printf (_("The help has not been translated to %s.  It will be displayed in English instead."), $languages[$squirrelmail_language]['NAME']);
     echo '</font></center><br>';
-    $user_language = 'en';
+    $user_language = 'en_US';
 } else {
     $help_exists = false;
     echo "<br><center><font color=\"$color[2]\">";