fixes #2557 use pwg_get_cookie_var

-Update cookie to use pwg_lang to avoid conflicts with other potential apps.
-Add cookie path to the pwg_lang cookie to help with conflicts also.
-Be less restrictive in load_cookie_language function
-We can't use pwg_set_cookie because it is set in js not PHP so we force it to be called pwg_lang

themes/standard_pages/template/register.tpl

@@ -111,7 +111,7 @@
     <div id="lang-select">
       <span id="other-languages">
   {foreach from=$language_options key=code item=lang}
-          <span id="lang={$code}" onclick="setCookie('lang','{$code}',30)">{$lang}</span>
+          <span id="pwg_lang={$code}" onclick="setCookie('pwg_lang','{$code}',30)">{$lang}</span>
   {/foreach}
       </span>
       <div id="selected-language-container">