When using a custom language path along with the messenger class, the messenger class only looks inside the default phpBB language folder.
So setting a custom language path has no effect on the messenger class.

Possible fix:

Open includes/functions_messenger.php

Find:

function template($template_file, $template_lang = '')    {        global $config, $phpbb_root_path; 

In-Line Find:

$phpbb_root_path

In-Line Add After:

, $user; 

Find:

$tpl_file = "{$phpbb_root_path}language/$template_lang/email/$template_file.txt"; 

Replace with:

$tpl_file = $user->lang_path . $template_lang . '/email/' . $template_file . '.txt';