I'm doing some research on a different phpmailer / BCC issue but since I had the same issue as this post, thought I'd reply with what I did to fix it.
The phpmailer package has three essential files:
class.phpmailer.php
class.smtp.php (optional - only needed for SMTP)
language file: phpmailer.lang-en.php (English althought there are a bunch of others)
In the phpmailer instructions (
http://phpmailer.sourceforge.net) you're supposed to add these files to the php.ini includes path. Instead I just referenced as includes at the top of my scripts.
PHP Code:
include_once("lib/phpmailer/class.phpmailer.php");
include_once('lib/phpmailer/language/phpmailer.lang-en.php') ;
I was getting the same error (Language string failed to load) until I added the second include for the phpmailer.lang-en.php language file. Once I did that, the mailer worked fine with no error messages.