Chouby on "[Plugin: Polylang] Compatibility problem with Subscribe2 plugin && Stop cookie language setting?"

That's not the best way to do with subscribe2 which is already internationalized. So you don't need to modify the plugin. You should just translate the plugin using poedit or the WordPress plugin Codestyling localization.

I did not make tests but had a quick look to the plugin source code and I believe that the strings are loaded before Polylang sets the language if you chose to set the language from the content. There should be no problem if you choose to add the language code to all urls.

Maybe (I did not test) this short code in a custom plugin could make the plugins work together when setting the language from the content:

if (!is_admin()) {
	global $mysubscribe2;
	remove_action('init', array(&$mysubscribe2, 'load_strings'));
	add_action('wp', array(&$mysubscribe2, 'load_strings')); // after Polylang
}

If it works, you can ask the subscribe2 authors if they font mind to hook in 'wp' instead of 'init' to load the strings.