Update Mail.php

I suggest to change the condition for including file_core to allow sending emails in multiple languages in a series. 

The way the code is originally, file_core is only loaded once and therefore, method argument 'id_lang' has no effect. Even if id_lang changes, $file_core is not loaded, only $file_theme. 

By adding the flag $_LANGMAIL[_ISO_CODE_] it is ensured, that file_core is only loaded when iso_code changes.