Language selection based on a domain\path hybrid

Question

I have a site translated in 2 languages - English and Spanish.

The English site is on example.com, the Spanish site is on example-SPANISH.com, so I set the "language detection method" based on the domain.

Now, the english site has a slightly different content based on the exact location of the users, so I have created different languages like EN-US, EN-GB, EN-CA, etc...

I want to set this (sub)language based on a GET parameter, like this

example.com/?from=england -> sets language to EN_GB
example.com/?from=canada -> sets language to EN_CA

I could use the Session detection method as primary one, but I want to prevent (for duplicate content \ SEO reasons) the spanish site to return the english content if the ?from parameters is set.

In other words, this URL

example-SPANISH.com/?from=canada

mustn't set the language to EN_CA: it should remain ES_ES instead.

Any ideas on how to achieve this?

I was thinking about a function like this in a custom module:

function MYMODULE_boot() {

 if(isset($_GET['from'])) {

 $allowedLangs = array(

  'www.example.com' => array('en', 'us', 'ca'),
  'www.example-SPANISH.com' => array('es')

 );

 if( in_array($_GET['from'], $allowedLangs[$_SERVER['HTTP_HOST']]) )
  $_SESSION['from'] = $_GET['from'];

 }

}

The problem is that when the ?from param is set in the URL, whatever i set in _SESSION seems to get ignored.