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/profile.tpl

@@ -481,7 +481,7 @@ const str_must_not_empty = "{'must not be empty'|translate|escape:javascript}";
       <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">