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Something like this should work:

Two files:

1. /includes/classes/observers/class.set_customers_group_pricing.php
<?php
/**
* @package plugins
* @copyright Copyright 2003-2012 Zen Cart...

A couple comments:
$_SESSION['languages_code'] is probably more user-friendly than trying to guess which languages_id to use.

It would be more efficient to hook the...

Simple. Do the same thing in the corresponding file that your custom addon uses.

When you created your discount groups in your admin, they were assigned a number. Use that number.

Open phpMyAdmin
Access the customers table structure
Edit the "default" value for the customers_group_pricing table to the value you desire.