Why do I get "Undefined index: mail en comment_form_validate()"?
From your error messages it seems you are calling something like this

$form['field_name']['en'][0]['value']


Or something like that.  Which means the array key "en" is not available.  If you are not using languages, then I suggest using LANGUAGE_NONE as your key selector.

To prevent these types of messages you should perform an isset on the array element to make sure it exists before trying to retrieve a value from it.